I solved this issue by myself as no one from 3 forums bothered to reply to my request for help. I can now receive full size MMS images on o2 (and probably any other network in the world) on my Nexus One. You obviously have to be rooted to modify the stock Mms.apk but it seems to work flawlessly. I've provided a picture to prove it. If anyone wants the Mms.apk I'll be happy to provide it.