Importing/Exporting Preview Signatures on OS X 10.10 (Yosemite)
It's a little too obvious now.  The image data is recorded as the password entry in KeyChain.  So, to copy a signature, create a signature on your new Mac by camera or touchpad.  Then:


Find the desired "Signature Annotation Privacy" entry in KeyChain on your old Mac.
Right-Click and "Copy Password to Clipboard"
Paste the text (base64 is all printable) into a text file
Copy the text file to your new Mac
Find the new "Signature Annotation Privacy" entry on your new Mac
Highlight and delete the Password field from that entry
Copy the contents of the text file to your clipboard
Paste the contents into the password field
Save Changes