Hopefully this is something that will be addressed in XF 2.0 as a part of a larger high DPI and vector based overhaul.
In the short term though, the biggest culprit with this at the moment (in my experience) is with screenshots uploaded direct from smartphones and tablets. They end up looking huge and quite silly when you view the thread on a desktop. A good example is this thread where both myself and MattW have uploaded screenshots from our phone.
Ideally it would be great if the system could recognise it came from a high DPI device, keep the original image and then css magic to scale it so it looks normal on a standard screen, but still looks super sharp on a high dpi screen.
I'm pretty sure I read Shawn has been using 2x scaling for his avatars on Digital Point ever since he launched on XF. I'm not sure how feasible it would be to implement this for mobile uploads, but I think it would be a great feature, and kind of necessary what with the shift to more people using mobile devices as a percentage of their computing time over laptops and desktops.