I've had my HP7520 for a year or so and just started having this problem.
I can print full page (8.5x11) photos with no problems. However, when I print 4x6 the photos don't feed properly.
Specifically, the photo prints, but the paper is not feeding along as it should. It appears that something is jamming the paper so it can't feed through or the feed mechanism just isn't grabbing the paper properly. So, I get about half of the photo printed and the rest is all printed on the top line.
I hear the printhead going back and forth, but the paper doesn't move until the last half or third of the photo (not consistent) and then suddenly feeds through. Part of the photo is printed and the rest is just a bunch of black ink all over the top and back of the paper.
I tried cleaning the rollers, but nothing changed.
Again, only having a problem with 4x6 photos. I haven't tried 5x7 as I don't ever print that size and don't have the paper.
Unless I get some big revelation as to how to clean internal components, I'm going to assume the printer has some sort of physical malfunction that can't be fixed and move on.