I'm hoping the photos I make on my computer will suffice... they're on nice glossy heavy paper and high color definition, so I can't tell the difference between them and "store" photos.
I guess some people try to just print them on regular paper, and I can see why UT would reject that.
Hint for those using PAINT: image attributes set to 225 by 225 will put you in the 2" x 2" neighborhood. Leastwise on a HP deskjet D2460 printer.