Red line on last quarter of photos

I assume that the red line is not on the photos themselves (make sure this is the case - if they're scanned photos, it might be a scanner problem).
If this is a 4" by 6" paper, set it in the printer after bending it in the opposite direction.
1) Set a sheet of paper on the printing side of the paper to be printed on.
(If the print surface is touched by hand, the print surface may be soiled by skin oils, and the print quality may deteriorate.)
2) To bend the paper, hold the paper in a rounded shape for about 10 seconds.
3) Remove the paper not to be used in printing, and confirm that, when laid flat with the print side up, the center of the paper to be printed on is raised between 2 to 5 mm (0.2 in.) .
4) The curled paper to be printed on should be set in the printer, and printed, sheet by sheet.

Canon recommends using Photo Paper Pro PC-101S 4" x 6" sized paper.
When using PC-101S, printing is performed at the center of paper and streaks or uneven printing is avoided.

If this is an A4 sized paper, try using Photo Paper Pro PR-101 paper, and make sure that the printing quality is set to "high" or "fine".

If nothing helps, I would suggest trying a new ink cartridge. If you still have this problem and nothing solves it, call a repairman.