Extract an image file of my signature from Preview?
You can alternatively try to insert your signature in a blank pdf document, crop to take only your signature and then save the document as a PNG file.

FYI, it's seems that the signature is stored in a .plist file.
The file name is "com.apple.Preview.signatures.plist" and it's in folder : /Users/YourAccount/Library/Containers/com.apple.Preview/Data/Library/Preferences/

Hope this helps !