Thread thumbnails

Could a mod please tell me what exactly are the requirements to set a thumbnail, because the randomness of what works and what doesn't is getting on my nerves. So i'm aware you can pick a thread thumbnail from the images posted in the opening post, sometimes you need to save the thread first, even wait a while, before the forum will recognize the images as potential thumbnails. This has worked for me in the past, however rarely flawlessly. Image can't be too big, can't be too small... but what is too big or too small? I'm currently trying to set the logo of Tour de Romandie as the thumbnail, there are 2 versions (one JPG, one PNG) of the logo in my opening post (pro cycling forum), both of different dimensions, neither works. "There are no thumbnails available to choose from."

Are there certain image proportions that apply?

Please lift the mystery of the thumbnail. Thanks.
Reactions: BlueRoads
I finally managed to get a thumbnail set up for this thread. At least part of the mystery is the following: the image in the opening post MUST NOT be resized, within the post itself. So no using the "change size" tool within the post editor, or no [IMG width="xxx px"] code in the post, to resize the image. AFTER chosing the thumbnail, only THEN can you resize the image in the post if needed.
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