For a few years we have included numerous images into our webpages, but never noticed much that they were picked up by Google or other search engines.
The images were included in between the text, almost always containing an ALT tag, and sometimes with a description below it.
The last half year, after we changed our templates, all images have been included into tables, one column containing the image, the other column including a description. Now we see steady increase of visitors coming through images.google . It maybe is a small advice, not meant for the experienced webmasters, but apparently putting your images into some separate unit that includes text and description does the trick (the ALT tag did not really)