Google cannot interpret images, so properly tagging photographs with metadata is the only way for AdSense to be able to tell what they are about and generate contextual ads.
Just like for text-based pages, the file name, the title of the page (i.e. the <title> tag) and the <h*> tags (for instance <h1>) are some of the most important things Google looks at. Google is also known to look at links, and for images, at their corresponding alt and title attributes. It is a good idea to place the keywords targeted by a photograph in most of these places, including the file name. On the other hand, uploading photographs from the digital camera straight to the website is a very bad idea. File names like "Image007" provide Google with no useful information to work with. Publishers should take the time to change automatically generated names to something that is relevant to the images they contain. Names should not be abbreviated - Google does not understand abbreviations, unless they are very well established. Properly separating the words may also be important. For instance, if a photograph is about the World Trade Center, call it "World-Trade-Center.jpg" or "world_trade_center.jpg" rather than "WorldTradeCenter.jpg", or worse yet, "worldtradecenter.jpg".
Adding text content is always a good idea. If a more detailed description of the photograph is available, using it will certainly not hurt, as such a description may well be the only sizable piece of text on the page! Some people claim that search engines give more weight to the first and last paragraphs of the text, and maybe to a paragraph in the middle. I'm not sure this is based on actual fact or is just another search engine urban legend, but it makes a bit of sense, and is thus worth keeping in mind.
Most search engines these days ignore the meta keywords and description. However, as long as publishers do not try to stuff them with unrelated words meant only to boost their site's search engine ranking (an exercise in futility), having them there cannot be bad and may provide additional information for AdSense (nobody cannot tell for sure, as Google keeps its algorithms secret).
Each photo album page will inevitably contain navigation links. Such links may include alt and title attributes containing the name of the next or previous photograph, or other metadata that is useful to create a better experience for visitors, but which may incorrectly influence AdSense's idea about the content of the page. To address this issue, Google recently introduced section targeting, a method of increasing or decreasing the relevance of certain sections of the HTML code in each page, by enclosing such sections within special comments. The content to be used as the basis for targeting should be included between the following comments:
<!-- google_ad_section_start -->
<!-- google_ad_section_end -->
while the material to be ignored should be included between these comments:
<!-- google_ad_section_start(weight=ignore) -->
<!-- google_ad_section_end -->
When I first set up ads on pages presenting individual photographs, I noticed that AdSense was serving photography-related ads (equipment, wedding photography, modeling, etc.) on pretty much every single page. This turned out to be due to the fact that the title of every such page was prefixed with "Tudor Hulubei - Photography -". Once I took both my name and the word "Photography" out of the title, AdSense started serving contextual ads. I was unpleasantly surprised though to see that my earnings did not improve significantly - the averages were only slightly higher than before. This seems to indicate that photography-related ads pay relatively well, and people browsing photo albums find them relevant. In some sense this is good news, suggesting that publishers may earn some revenue even if their photo albums do not include enough metadata for AdSense to be able to serve proper contextual ads, but include enough to make it believe the album's main theme is photography-related.
Going back to navigation links, it is obviously important to keep people from leaving the website. The longer they stay, the more likely it is that they are gonna click on an ad. One suggestion for keeping people around longer is to include on each page links to a list of the "most popular" photographs. People are naturally curious, and they will oftentimes be tempted to take a peek at what other visitors found interesting on a site, so chances are they will click on those links (a traffic monitoring tool such as StatCounter could prove very helpful in determining this). How one defines "popular" is obviously subject to interpretation. Some interesting choices, with their associated "evil" rating, are the photographs that:
- have the highest hit count (honest)
- publishers consider their best from an artistic standpoint (narcissist)
- publishers want to draw attention to (entrepreneur in training)
- are believed generate the most revenue (hard-core capitalist)
Simply including a high-paying, high-CTR photograph in the "most popular" list does not guarantee increased earnings. Realizing that most visitors come through search engines may help you understand why: when somebody looks for an image of Bucharest on Google Image Search and is directed to Hulubei.Net, that person is probably not interested in images of flowers. My tulip photograph may have a high CTR, but that was computed by averaging over the number of clicks generated by those people who came to my site looking for an image of a tulip, rather than those who found it on the "most popular" list! If anything, including the tulip photograph there may significantly decrease its overall CTR. Perhaps a better strategy for that list would be to include photographs that are closely related to the current one, but I have not experimented with such a scheme at all (well, Amazon has), and I assume setting it up would involve a configuration far more complex than what most people would have the desire to implement.
With the same idea of keeping visitors from leaving their website for as long as possible and squeezing every bit of advertising revenue out of them, some publishers have had some success replacing ads on low paying pages with links to those pages generating the highest earnings. This pushes the "most popular" idea a step forward, so even though I do not have any direct experience with this technique myself, I thought it was worth mentioning.
At the end of the day, publishers have to accept the sad fact that their visitors are eventually gonna leave. When they do so, it might be tempting to try to make them leave through either a link to some other photograph or through an ad, by simply making sure those are the only choices available on the page. However, remember that a good site is about content! If visitors get directed to a website in their search for an image of Bucharest, then providing a link to Wikipedia's section on Bucharest or some other interesting resource may improve the visitor's experience. Of course, external links can always be opened in a new window or tab, ensuring visitors will see the original page one more time, even if it is just to close it.
Finally, something as simple as a search box can both improve the visitors' overall experience and give publishers an idea of what their guests are searching for, while at the same time bringing in some additional revenue. Google's AdSense for Search nicely complements AdSense for Content - publishers get paid every time somebody searches for something through a search box on their site and clicks on the ads included in the search results. While earnings for AdSense for Search will be minuscule compared to those from AdSense for Content, for pages made up of text-based content, the former is often reported to have much higher CTRs than the latter (10+% vs. 2%). Google's Terms and Conditions prevent me from disclosing the CTRs I see for my site, but we all know that a picture is worth a thousand words.