It really depends on the situation. I wouldn't necessarily jump to converting the signature over to an image and store it separately (more so than it already is). If you will be displaying the signature often, you could look into Fragment Caching as a possible solution for the areas where you're displaying the image. Converting and storing the signature as an image seems like unnecessary overhead as well as an added complication.

However, if this is the route you need, you could create an attr_accessor for the signature and convert it to an image and upload it like you're describing.