Hi, I don't know what your portfolio looks like, but if you check out most of the images for sale on Cafepress, the photography tends to be fairly amateur. I think it has to do with the fact that most people are there selling other stuff, and they may be great designers but only put in their photography as an afterthought.
If you have a portfolio of professional quality images, and you can come up with creative and visually appealing designs for them, it may actually work. Certain categories of stock photography fit this model very well for the kind of junk they sell there, if you can do this, and you will have an advantage because most of the people there would need to pay more money than is worth the effort to license professional quality imagery to resell on their products.