I have a client who has run an seo campaign based on providing logos to other websites, with alt text containing his keywords, pointing back to his site. This was implemented before I came on board, and my first inclination is to cease it, as it seems waste of time. Anybody out there actually have success with this? It seems google is smart enough to see that any alt text should be given very, very small weight as a link (these are not text links...just image links) He is ranked ok for his keywords, but I think this has more to due with the age of his site (8 years) and other (natural) links he's gained over the years.
While it's probably not the most effective way to help SEO, I'm sure adding ALT text to an image is helpful and should be done. I'm pretty sure SE's look at the ALT text for an image link and puts the two together. If it will help, why not, right? I don't personally know how effective it is though, so I can't really help out with the topic question!
I always put alt text in images as part of my onpage optimization strategy. It's a good way to make your images searchable in google image. Alt tags are created for robots.
If you have an image link like <a href="URLofYourSite"><img src="URLofImage" title="some hover text" alt="Some Link Text" /></a> then the alt text is considered by Google and most search engines to be the link text or anchor text for the link... It generally carries a little less weight than a 'real' anchor text but in the absence of a 'real' link text, always include an alt attribute. Remember the alt attribute is really for accessibility and user agents that have images turned off. The alt text is supposed to describe the image so screen readers can read a description of the image to the handicapped. It's not supposed to be stuffed w/ keywords you want to rank for. But if you can work keywords into the alt text while describing the image at the same time, that is fine.