n900fan75,
Just to make suggestion to you my dear and beloved friend.

Have you tried to change fmms picture size to small. Because if some phones are able to send mms messages without proper data plan on Rogers then it means that it´s probably possible to send small amounts of data at once and this means that maybe perhaps fmms default message size is bit too big. I would test this by trying to send really small pic with message let´s say 128x128 pixels.

Please help yourself!

.edit
And btw. I assume that you have done your homework and least checked that you have these settings:
http://wiki.maemo.org/MMS#Rogers