You don't actually need photoshop exclusively to put a texture over your artwork. You can gain the same effect on Paint Tool Sai by pasting the desired texture on a separate layer over the art, clipping it, and then setting it to 'overlay.' This is a really nice trick, though, and it can be helpful to a lot of different artists!
I LOVE all of your stuff. It's so incredible and beautiful, but somehow so unique in it's style. I can't wait to get some time off from college to practice some of these (I really need to practice SAI).