nomis5 - 8:04 am on May 16, 2013 (gmt 0)
I don't think G can recognise a finished recipe picture from one that is not just from the picture. But it is very clear that they almost always show pictures of completed recipes.
I went into this in lots of detail a couple of years ago and it seems to me that there are two key factors. First the alt tag must clearly identify what the picture is of. For example if the recipe is for beef casserole then a picture with the alt tag "beef casserole" will definitely help G to identify it as such.
If you have multiple different pictures, make sure only one has the alt tag "beef casserole". Other pictures should be clearly distinguished as not being of the subject. An example might be an alt tag of "mixing flour and water for the gravy". That way G is not confused as to which picture to use.
It also seemed to me that a key factor was that the image should have size properties - no size properties, no thumbnail.
Is it all worthwhile, because yes, it requires lots of effort? I think most definitely it is worthwhile. So many recipe sites have no pictures because they have just ripped of the recipe from another site. The public is wise to that and love to see what the finished article will look like.