ComedianGR, you are missing two things:
1. you're mixing up filesize with image dimensions - you can have a small image with alot of kb (like uncompressed bmp), as well you can have very huge images with almost no filesize (like 2048x1200 pixels, all blank white with png compression). Of course there must be a limit to image dimensions! But not to filesize, since:
2. everyone, just like TTD, can turn off images within signatures.
There really is no point in limiting the filesize, as long as you declare uncompressed formats like bmp invalid and provided that the images are hosted elswhere and not on this server. Of course pngs should be maximum compressed, that's self-evident.
But let's not discuss too much about this. People will get edgy from off-topic
I didn't think there are such weird rules, when there are options to just disable the images once and for all, so I tried to help Freya Nocturne on the topic to get better quality. Just come close to your display, compare these and you'll know why I said so:
[ external image ][ external image ][ external image ][ external image ]
[ external image ]
But it's impossible to do so, as Freya stated correctly, because any png would exceed the allowed 50kb rule.