Fotolia's categories are a bit weird - back at the beginning I thought eventually they'd expand them to be more complete, but (as with a number of other site functions) that never happened.
There is a browse by category, but I don't think that categories play into search results. I do not believe you can submit without making at least one choice.
Given the way the submit process works, it's pretty easy if you have a whole batch of images in the same cateories - pick them once and then they'll just be there for all the other submissions in that batch. They had a good idea with the "most recently used" lists for categories, but that's been broken since version 1 (perhaps even in the beta before v1 came out, but I'm not sure), so you can't use that.