Image SEO Basics: 1. Alt Text, 2. File Names, 3. Surrounding Text.
Adam Rifkin stashed this in SEO
SEOmoz has a great video on image SEO:
Let me go over some of the important factors with image SEO. Number one, I already mentioned this a little bit, is alt text. Alt text is the text that you provide for an image in case it can't be displayed. Maybe the image is gone or maybe someone is using a program that can't display images. This is the text that takes it place. So it makes a lot of sense from an SEO perspective that this metric is going to be important because it's the information you tell the search engines and other technologies what the image represents. With these, I recommend keeping them below about 140 characters. It's a rough rule of thumb. Also, have them be descriptive and in line with what you're trying to target for that page.
Number two is the file name. This works off the exact same principles. The file name is also information you give directly to the search engines and to other technologies to identify what the information is about. I would gander, if you will, that the file name is probably a rougher signal than the alt text. Alt text, from my experience, when it's there, which is not all the time, in fact, alt text is not included many times which is bad for SEO. But when it is included, it tends to be a clearer signal than a file name which a lot of times is just algorithmically generated by the timestamp, so it's just a bunch of numbers.
Number three is the surrounding text. I think a lot of people don't think about this when they're thinking about image SEO. The text around an image tells a lot about the image itself. This makes sense, right? You'll see a lot of times where images will be on a blog post and you'll have a caption describing the image. This is just another signal telling the search engine and other people and technologies what it is this image is about. The surrounding text, and that can either be a caption, like you've seen traditionally, or it can just be the paragraphs around the image. A lot of times an image will be used to supplement what the textual information is talking about. So the surrounding text is very important.
Fewer images per page is a good rule of thumb, too.
Search engines tend not to index entire galleries of photos.
So to summarize: Alt Text, File Names, and Surrounding Text.