I remade the X-Mod icon with a fancy 3D letter 'X'. You can use it for the mod thumbnail if you like.

https://drive.google.com/drive/folders/1FjWR0piyq9NdlTWKChnqS4KLMszqaNtC?usp=sharing

Includes:
PNG thumbnail at 180px
PSD
TXT image credits
JPG in-game preview

Why is the thumbnail 180px? An unnecessarily long explanation:

Most thumbnails are 144px. Any higher, and the mod portal will generate a 144px version of it to show on the mod portal, and in-game while previewing and installing the mod. The 144px icon looks great in a browser, but VERY bad in-game on interface scales above 1.00x due to a bad upscaling algorithm. The high-res thumbnail will only appear in-game after the mod is installed.

A thumbnail size of 180px scales pixel-perfect for players at interface scale 1.25x, which I assume is a plurality of the community. (I play on a TV at 1440p, so my interface scale is 1.50x)

A thumbnail size of 720px looks kinda jagged, but decent, at any interface scale. However, on your mod's thumbnail, the diagonal X was too jagged at 720px.

Ohhh that's very cool, thanks a lot! And if I want to swap out the background pics at any point that should be easy enough to do with the psd file you posted right?

FYI the spidertron pic you used was taken by me anyway :) It's the thumbnail for https://mods.factorio.com/mod/SpidertronEngineer and I provided Alt-F4 with the original res version for them to use.

But given that it's a thumbnail for a mod not in X-Logistics/X-Mod I think I'd rather use a different image. I'll try and make a suitable one for Spidertron Patrols I think.

Awesome, I'm glad to be of service, and fascinating that I accidentally ended up with one of your pics!