Don't be too harsh on Brad - he's a busy guy. :)
There used to be a link to art submission guidelines in the FAQ, but it has recently been rewritten, and that helpful little tidbit has been lost.
Characters are transparent GIFs, technically size-limited to 125 pixels wide and 200 or so pixels high, but it is best to keep them within 115x180. When you make them, you have to make sure they use the smallest possible number of colours - file size is important.
Backgrounds are JPGs, with minimum dimensions of 250px wide by 350px high (the size of a comic panel). They can be a bit bigger, so there are no ugly borders when the frame is stretched by extra-large text boxes, but this makes the file larger as well.
If you can't work the formats out, or want the double-checked before submitting them, feel free to send them to me - mcdexx@optushome.com.au - but try to keep the email under a hundred KB or so, please.
Hope this helps...
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