Since there are a number of threads on problems with sending/receiving MMS messages (that contain pictures), and since I just spent some time resetting my APN configuration by hand, I thought I'd point out that there is some disagreement between different support pages on this subject. For instance, page:
Has the User Name set to <Not Set>, but the page:
Has the User Name set to the literal string 'None'
Turns out the later page is correct, the User Name must be set to 'None' for the APN settings to work correctly.