As both threads point out, its important that your alt tag (alternative attribute) is descriptive. Also, it helps to name the file with the keywords in them, example given at JimWorld "a picture of a car should be nissan-maxima.gif with the lat tag being Nissan Maxima."
At SEO Chat some members believe that "putting each image in a specifically optimized html page" also helps.
Do you really want traffic from Google Images? Will they convert? That is your call. Just make sure you don't get hit with high bandwidth bills that make the ROI in the red.