I (MechanicOrga) created this template for people to add pictures of their original monsters to if they so please. The aspect ratio may differ from the in-game singleplayer thumbnail but it works.
To use this image you place the monster on one side (from the waist upwards) and their logo on the opposite side. You can also crop it to 1024 x 1024 for the multiplayer thumbnail or a head icon. This can also be used for YouTube thumbnails or other such projects.
Be sure to mention I made this if you do; I'd be most appreciative!