Once I tried "freelancing" as spec at 99designs but it didn't got to a decent income/working time ratio.
Asked because wanted to know if it would worth the effort migrating/posting images straight to such stock sites instead of using openclipart. There, having about 2345 images myself, getting the comment "get what you give" I think free giveaways are not respected -or rather not valued enough since they don't have any price tag...
I'm interested in the artistic side as well so guessing stock sites may not be what I should follow.