Just a first stab at that.
-- Creates a single use coupon rather than a gift certificate
-- Attaches a jpg image of the voucher to the order confirmation email.
-- Adds the code and the value to the jpg image automatically
That's about it.

It is rough around the edges and could probably do with a configuration page but if someone would like to test it the way it is then that would be nice.

The zip is a bit big for the forum so it is at http://tinyurl.com/c4uwflu. (It is just big because it contains a font for writing the text to the image - the rest of it is just a couple of files! )

Thanks