I have one site that only has about30 images on it and they are all alt tag labeled and file named to describe what they are and I get some pretty irrelevant traffic that way. On one site I would say that it even runs as high as 50%.
i get 70% visitors from Google for my php blog and none from msn... 10-15% yahoo and others from social bookmarking sites... Thanks...
Since my site doesn't have more images, the percentage of visitors through image search rarely exceeds 5%.
One of my blog gets nearly 75% from google images. Thats why Google adsense showing only public service ads in my blog. More traffic no revenue...