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Google tends also to go for finished dishes, rather than for images of preparation
A lot of the answers lie in Rich Snippet markup for recipes. See Rich snippets - Recipes
In the recipe thumbnails pulled, Google tends also to go for finished dishes, rather than for images of preparation.
I don't think G can recognise a finished recipe picture from one that is not just from the picture....
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.
I'm seeing that Google has gone in and cropped a shot that was too wide as a thumbnail but was otherwise clear graphically.