Lumia 900 Trouble receiving hi-res photos via MMS ?

Is anyone else experiencing this? I can't seem to open MMS messages containing photos taken with iPhone 4s' or Galaxy SII's (which is to say, photos taken by at least an 8MP camera). I'd imagine this is an AT&T problem, but I am having a hard time wrapping my head around the idea that AT&T is restricting its customers in this manner (especially those with 5-bar LTE!), especially considering sending photos of this size through iMessage is no problem at all.