Hey folks,
today I saw the map "Astro Crater Battles v5", which had weird thumbnails in the map vault. The small preview image (usually <map>.small.png) has actually a size of 571 x 578 pixel, instead of the 100x100 FAF seems to use. So it looks like there is the possibility to actually upload a map with incorrect thumbnails.
So I investigated a little bit further (uploaded one of my maps with premade .small.png and .large.png thumbnails with a single black pixel in the upper left corner) and in fact it is possible. Both the small and large versions are used directly from your map directory and are not generated by FAF when you upload the map, if they already exist. Also you can see that my large version has no markers in it, so it was clearly not generated by FAF.
Conclusion: You can just make your complete own thumbnail for the map vault, which could possibly fill your whole html table cell.
Because I guess it should not be possible to actually bypass the FAF thumbnail generation like this I would consider this as a bug.
Regards,
Jilo
PS: Please don't abuse this to cover the map vault with large, useless thumbnails.