I thought that the screen size on the V3c (which I just got) was 240x320...I've been having the same issues with wallpapers, the default ones fit so perfectly, buy anyone I send with MPT has the left/right edges cut off, so there are strips of white on both sides of the pic -- very annoying.
Also, MPT will automatically shrink the file to 6k if I transfer it in the images section. I have to use the file transfer wizard to keep them how they are, but then quite often I get a "fille is too big for phone to display" error.
I'm working on it, and when I find out the best size, I'll post my results.
*EDIT*
For my RAZR V3c, I have found that an image size of 176x220 and the gif format works the best for me, perfect size, no white edges, and best picture quality.
I take any picture, and use acdsee to resize it to 176x220, save as a gif, and then use the file transfer wizard on MPT to transfer the pic to the phone.