Sounds like your running through a net accelerator of some type, these work by compressing internet traffic to make it smaller so your connection seems faster. one of the side effects is that pictures are shown in low res!
You may be able to switch this off in a setting somewhere in the dongle setup/properties, but it may also be done on the provider side of things in which case you can't do anything about it your end.
have a look through the settings for the dongle, see if you can see anything about turning of acceleration or compression, if not then contact t-mobile about it.