I use "alt" tag for every pictures posted on my sites as well and also got some traffic from that, although it not much but not too bad.
Google (and other search engines) are much slower to index images than pages. It can take several months (Don't be surprised if it takes half a year). You just have to wait. Of course if your site does not rank very well it will be more likely that the images never gets indexed but that holds for pages too.
google also considers the content on the page to index your images... you can have images that are not related to the search term that the person used, but on the page where that images are located you have a text with the proper terms...
