I don't think G can recognise a finished recipe picture from one that is not just from the picture....
You don't think the algo tastes the pictures along the way? ;)
In the news image thread, several comments suggested the image to be thumbnailed should be at the top....
Keep the image near the top of the article. On our site we put the images right next to the Headline and the story summary, with the article appearing below the image.
This appears to be part of the formula among the images I've seen returned. Does the presence of additional step-by-step photos make Google less likely to pull from that page... or do the alt tags and position on page suffice?
Results I've checked suggest that if an image is too large, Google won't use it as a thumbnail.
PS: Interesting to note that the thumbnailed results on some searches I've been checking are not static... and I'm seeing that Google has gone in and cropped a wider shot that was too wide as a thumbnail but was otherwise clear graphically.