I'd like to build a letter module that includes a handwritten signature to be stuck onto the end of the letter.

Here's the catch.. the same clients will receive several letters and I'd rather each letter have a unique version of the same signature.

One thing I was thinking was.. Scan 30 signatures, and choose which one to use randomly per letter generated.

That would be a big step toward making each signature unique..

is there any other way ?

No, I think you're on the right track. You need a table with scanned signatures and randomly pick one (or cycle through them).

Probably better to cycle through them, as if it's a random from the whole pool (eg of 30), there will be a chance that successive letters will pick up the same version of the signature. :)