I've never used Polyvore for outfit creation, but do visit the site regularly for inspiration: either to see which combinations people come up with for a particular item, or to see what they are doing with a new trend.
I believe several YLF'ers use Photoshop to edit their pictures and put together collages? Before posting a pic, I also quickly run it through Photoshop to adjust the lighting and crop an image. Only the very basic stuff, though, I'd love to learn how to use it properly. (it's on my to-do list;-)