Emailing photos in iOS 5.0.1
The problem can be in 1 of 2 areas.  Either there is a problem with the photograph, or it is a problem with your Mail setup.

You can test both areas, try sending a different photo, see if it works.  Or, try sharing the same photo via another mechanism, e.g. messages, or Twitter, see if that works.

If one of them fails, then that is your likely cuprit.  If they both work, then it's likely to be a little more nuanced and harder to track down!