appears that it's pinch to zoom..? Probably a new stock feature and not a bug - noticed it on a MMS I received. Kind of nice because you can blow up the thumbnail and don't have to d/l or put the pic in your gallery. Just pinch it back to your preference of size.
Are you sure you are not accidentally pinch-zooming? It is a new (AND WONDERFUL) feature in Sense 4 for so many additional places, including text messages.
The other fantastic new feature is, FINALLY, the ability to set your font size for most of the Sense UI, and messaging! No more looking for glasses all the time!