It could be your provider. In the netherlands, t-mobile has an option (or at least had, I don't remember seeing it lately) to scale-down images in their proxy. As all your requests can go trough their "image" proxy at a click of a button (probably something to do with your APN, most likely server side even?)...
So it might not even have anything to do with Android..
It might be, but I don't remember if it was in my case, the Web 'n' Walk Accelerator
There's some google hits on that, though most of them semi-old. On this link some tips on how to fix it with that webnwalk stuff, probably not what you are looking for, but also some hints on how to send an "always reload" message to the proxy. Maybe that helps. Otherwise you might need to google some on tmobile and proxy/accelerator.