Hehe that is a very dedicated feature request, thanks a lot for it.
I didnt dig into the different modes for background-size and in general this scaling stuff seems to be one of the dark arts in html. We come from fixed pixel size but i was not happy with how it scales.
Also i was not completely satisified with your proposed "contain" alone because the images looked a bit less sharp on my main Screen (UHD, 1,5 times scale factor).
So i decided to convert all of the icons from png to sgv (we finally have an official svg ffastrans logo now as well
) but also i changed to "contain" as you proposed. The changes are done and pushed to github but of coruse a new build will take a while, we don't want to update releases every day