While completing an accessibility test on my website I also noticed that some search engines ignore you if you use one keyword excessively in your alt tags. (i.e. more than five times it assumes you're trying to fool the search engine.)
In a world without walls and fences - who needs Windows and Gates?! - Unknown Author
"And there's Bill Gates, the...most...famous...man in the...ah...Microsoft." -- A TV commentator for the 2000 Olympics.
I don't see what's wrong with that, as long as your still accuately describing the image... i.e. on a site about dogs, putting alt="Dog, breed: ****zou, age: 4 years old, white fur, ****znous caninus" or whatever.
Yes the ALT tag does help you get a higher listing on the search engines. It's best to match your keyword where you can but obviously don't over do it cause the search engines don't like that. Also it's important to keep the keywords closest to the front of the text (if that makes sense). IE if you were selling milk bottles and your keyword was 'milk bottle' don't write the ALT tag as "picture of milk bottle" write something more like "milk bottle picture."
A number of things help search engines find you. It's best to do as many of them as you can (meta keywords/title/description etc). So yes, optimising the ALT tag will help towards getting you higher up on the engines but you must also optimise everything else in your site too.
I always use ALT tags to describe the pic, but they can also be optimized within reasonable limits.
If you have images which are relevant to the page, and each page is also optimized then you will get a boost in most engines.
So ALT tags on their own aren't going to get you high on the SEs but they will help.