The Certificates module for Moodle will allow granting certificates based on completion of a Moodle activity.It allows uploading graphics and setting the style of the generatied certificate. Once a user has met the minimum criteria they can download a PDF and print their certificate.

There are several options for setting criteria for granting a certificate. These include overall course grade, time spent in the course, and ability to require one or more activities be completed, optionally with a minimum grade for the activities. Any of these requirements can be combined to determine when a student receives a certificate.

 Images can be uploaded to style the borders and include a watermark if desired. A signature image can be included, and of course, a logo can be uploaded.

The course administrator can choose to display the grade and, or various learning outcomes onto the certificate.

The certificate module is well documented in the Moodle Wiki.