You need to upload it as artwork and do some tweaks for the size. I think I followed this guide, but I'm not 100% sure since it's been quite some time:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=748624905

There might be better guides nowadays, I didn't check, but feel free to do so - Aevoa is the guru of all things profile-related (and he has tons of guides for various profile-related configurations, you can see his name in the collaborators list of the guide linked above).

...hey

The pro