Pics that are roughly square (or can be cropped to be square) or are wider than they are high would work best since a banner is rectangular.
The best way to give me specific pics (I can go and find some if you don't have a total preference) is either post a link to the image or use the same code you use to post a pic to a journal to actually include the image in a comment ([img src="image address"] only with pointy brackets not square ones). Simplest is just paste a link in I suspect.