Signature images are embedded in an XML file within the database. It's possible that the SQL upgrader didn't handle these properly, resulting in the image just dissapearing.
I'd recommend trying to re-apply the image in the Administrator Tool. This can be done by signing into the Administrator Tool, locating the affected bitmap stamp, and selecting the '...' symbol on the Bitmap portion. I've attached an image of this as well.
Software Support Analyst
Team GREEN Americas