Hi Paul I have only done this once so apology in advance if this is not very clean or I get it wrong.
When I edited our photos I saved some with borders at 7 x 5 and some at 6 x 4 since I needed this site for the troops autograph books.
Say you had 100 photos at 6 x 4. If you save them all at 7 x 5 you will then have 200 photos put on the pre-paid photo cd

The originals and then the 7 x 5's.