You'll need to draw them to fit 125x175px, and they'll need to be transparent gif's- there's a tutorial on that here if you need it (in the second post).
Once you're done you'll need to e-mail them to brad for approval, if he likes them, feels they'd make a good addition to stripcreator, has time and can be bothered he might add them to the site.
Got any samples?
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Dinosaurs had eggs bro, the chicken came way later.