You can keep a copyright sign in the image if that is your preference, but obviously hopefully not in the way that obscures the image. If it helps, we only need a relatively small image so from a "someone stealing it" point of view, it will hopefully have limited use.
Any organism would be lovely. Essentially whatever you have or whatever you are interested in is great. If you're looking for requests, we're particularly low on microbes, plants and insects right now. We'll also be looking for images that would depict some sort of environmental challenge/action as well as habitat type cards.
How realistic the animal is up to you (there's a thread on this somewhere in this forum). Basically, from testing with a variety of children (ages 6 to 14), the preference for style seems to be all over the place. However, they did like colourful, and the students knew when the art was of "high production value" versus a quick sketch kind of thing (you can peruse the current submissions to get a feel of the variety we have there). Anyway, we're going for a variety of art styles at this point, but could entertain specific art style sets in the future?
If you submit to Flickr in other dimensions, that's o.k. too. Just makes it easier for us if everything is ready to go at the requested dimensions.