A couple of things to check:
1) Be sure your scanned signature is done with a medium ball pen. Really fine ball points don't scan a well IMHO
2) crop the scanned signature to minimize extra whitespace around the signature and then
3) set the container field properties to reduce or enlarge the image to fit.
Extra whitespace is considered part of the image and the container will by default shrink the image to display it in the container. Removing the extra whitespace allows the image to show up with minimal resizeing (or even resized larger if the container field is made bigger). This allows you to size the signature to fit the document in your layout.