the no-external images thing I think should refer to objects like other cars, buildings, scenery etc. Like gtuned said, a lot of work and brushes are done by using what is an image to create an outline or brush and effects, but aren't actually adding the image itself.
The artsy entries I don't think ruin the competition, with an actual theme too much art might cause an entry to lose if it detracts from the theme. I personally prefer the focus to be on the car, and don't like lots of effects. Since the winner is up to the voters, I think we allow the rules to allow that kind of stuff. If the voters keep the theme in mind when they pick a winner instead of just picking the best edit, the artsy stuff like the weather edits shouldn't show up week after week.