lucy24 - 8:48 pm on Sep 23, 2013 (gmt 0)
and i know that they can recognise what's in a picture, because google images can do that even now. if you stick a picture in the search box then it will tell you what it is. so any search engine should be able to tell whether the images on a page are relevant.
The conclusion doesn't necessarily follow from the premises. What they recognize may be "line drawing" or "something bright red in the top right". Not everything is as unambiguous as a picture of the Eiffel tower.
I once experimented by dragging in a photo of one of my pet rats hiding in a small paper bag. Some results were other people's rats. Others were pictures that had a dark thing in the middle (my rat) surrounded by a white or light-colored thing (the bag).
Now, if you could do a combined search for "looks like this" (your dragged-in image) and "is a picture of" (page text, especially the alt) ...