Usualy blogger gets the first picture in the post to use as thumbnail for various widgets.
Sometimes it picks the second picture or whatever.
Is there any way to specify explicitly the picture to use as thumbnail, or submit the thumbnail to be used myself?
Best Answer
Google has never shared the formula for how they choose the thumbnail with us.
However various experiments make me believe that it's the first image over a certain size (perhaps 20k?) which is hosted in Picasa-web-albums/Google+Photos.
I haven't heard or noticed restrictions based on file name - but that's not to say they don't exist.
I documented what I found when investigating this here: http://blogger-hints-and-tips.blogspot.co.uk/2011/07/postsnippet-and-postthumbnail-tools-for.html - but that was in 2011, and there are no guarantees that things have not changed since. I've seen a claim somewhere (but not verified it) that the need for the photo to be Google-hosted was relaxed at some point.