My competitor's website gets excellent traffic from google image search. But not one of my site's images comesup. I put the keywords in the file name and also put alt-text for it.
How about the web content which relates to the image? I believe that is another point of consideration to work on.
alt tags = success + related info to your images I have about 300 UV on one of my WP blogs and 21 % of my traffic is from Google Images .
Alt tags, plus google has another way now to get more advanced results in relation to you images. Check your google webmaster tool account for the appropriate code.