I am running wordpress sites and I just make sure each image as its post title as alt text. Did it for about two months. Now gg image referrals have outraced that from gg search.

The plugin you need for that purpose is named seo-image:

http://wordpress.org/extend/plugins/seo-image/

The only challenge on using this trick is on those post without an image. I get around it by inserting stock photos from a close subject theme. And then it incurs a task of building an inventory of stock photos on different subjects LOL.