I believe you misunderstand how it works. The user has to have downloaded the image either from a server or otherwise, in which case they have probably already downloaded the map anyway. It's not something you can just make public for them to see to draw interest.
...you can't. People who already downloaded your map won't automatically download the menu thumb image. You can create a .res file for future releases of your map and include it there so the images will be downloaded together with the .bsp file. The .res asures that TF2 knows which files should be downloaded and can be used for files that can't be pakrated (like intro videos and those menu thumbs).
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble