My thoughts are it should be a fabricated part, and not a part that everyone in FIRST received in the kit or a full robot.. (I'm serious, keep reading
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A good teaser pic can have a kit part incorporated into it (re: subassembly) but not just a kit part.
Also, don't take a picture of the kit-bot all assembled and say that is your teaser pic. That's lame.. Every one got the same kit. I think we have enough pictures of them.
A picture of a custom fabricated part that your team put effort into making (more than a simple pillow block or wheel spacer) would be cool..
Examples of these different scenarios could be, Swamp thing's body from this year, The crazy custom Omni wheels from 2003, and last year people were showing, or the picture a few years back (someone find the link) of a full robot being transported in the backseat of a car, but you couldn't tell what it did by the angle the picture was shot at.
Also, I have to admit that Tom and team 195 has some cool teaser pics this year. It is a picture of the robot, but it it blocked out in a solid color, so you can see the basic shape in it's square blocky old school NES graphics way, but can't tell what it is. And the best thing about it, is that it is very simple to do even in MS paint. AHHH What is under that grey block????