Inside AdSense: A picture isn't always worth a thousand words
Google Adsense Ads and Relevance
Google Adsense team helped users understand how to get higher quality and more relevant Google Adsense ads to display within your content:
1. Use text in addition to images when possible. Please note that the crawler can't read text contained within images.
2. If you use images, be sure to title them appropriately and provide descriptive ALT tags.
3. Try imagining your site without pictures or dynamic content. This will give you a good idea of what our crawler reads. If you lack imagination, you can try turning the images off in your web browser.
If your site uses a lot of these features, make sure you provide enough text-based content in one of our supported languages to effectively convey the topic of your site. If our crawler doesnt find enough text, you may get poorly targeted ads