Android - MMS images showing up as videos

This might have something to do with a feature of MMS called Content adaptation. Your operator's MMSC (Multimedia Messaging Service Center) determines if the data sent is in a format supported by your handset, and if not, transcodes it into another format. This can cause some problems between different devices and operators. For example, my Android receives single images from other Android phones as 10 second videos where 1st half is the image and 2nd half is the message.

At least here in Finland, the behaviour I described is pretty standard and has happened since the days MMS was made available and people are used to it (as far as I know). Don't know about AT&T, but I'd suspect there won't be any changes in the near future.