Robert_Charlton - 3:14 am on May 16, 2013 (gmt 0)
DXL, your question reminded me of a discussion we had several years back about getting Google to pick up thumbnails in its News results....
Google News help - how to get thumbnails in news feed
I made some observations that the images used tended to be tightly composed close-ups. There was discussion in the thread about other image and page properties that appied to News thumbnails, which might also apply to recipe thumbnails. Size range of the original image, eg, mattered, as did position on the page.
Because recipe photos, by their nature, tend to be close-ups, I thought, therefore, that closeness might not be a distinguishing factor. I just did some recipe searches, though, and see that the thumbnails used were pulled from what I would call the most graphic and tightly composed of the images used on recipe pages. I'm guessing that photographic factors, like lighting, which might affect popularity in Image Search, are also thumbnail factors.
In the recipe thumbnails pulled, Google tends also to go for finished dishes, rather than for images of preparation.
The above noted from brief searches only, but I'm thinking it would hold.