Long text messages, photos - all show up as a five second long "video" (still image). Happens with received MMSes and the ones from me that I've sent. Using a galaxy s2. This is really annoying, my shoddy old nokia brick just opened them up how they were meant to be and then offered 'save to gallery' when I left the text. Why would such a better phone use such a worse approach? At first I thought it was that for some stupid reason it couldn't read normal pictures, but it does the same with my sent messages so unless it converts pictures out of readable format and back into a still video, that's not it (and that was also the only answer I could find on google).
I agree with Onik's comment linking to his answer to a previous similar question; this very well may be up to your carrier and something you'll have to take up with them.
SMS/MMS delivery is a ballet involving the sending and receiving carriers' interest in cooperating, the network technology your phone uses, and how the SMS app on your phone interprets the messages you receive. An unfortunate number of these factors are out of your control, but have you tried another SMS app like Go SMS or Handcent? These may help your issue.