If you had an image of a signature on a white background you could make a selection of the background and then clear it? Does that help?
How do you select the background?
Sent from George Mitchell's iPad
The easiest way is probably to select the Magic wand (fourth icon row on the tool selector).
Keep it pressed if not visible and select it in the expanded row.
Now tap on the part of the layer that does not have the signature.
Once selected you can delete it by pressing the Edit menu (pencil icon on the top bar) and press "Clear".
Hope it helps,