An ASCII document can't contain an image, since then it wouldn't be ASCII any more. Perhaps I'm missing something.

If you just want to sign an ASCII document (i.e. you don't have to have an image of a written signature, but you do want to authenticate the signer and validate the integrity of the message), you should look into Crypt::DSA::Signature or a similar crypto based signature mechanism, which can attach signatures that are themselves ASCII text.