Because files are overwritten when Processmaker is updated, it would be more efficient to create a custom skin for your application, and then customize the login information for that skin accordingly. Then you can export the custom skin to a tar file for later importing into a new or updated version of Processmaker. It's a bit of work up front but well worth the effort in the long run.
For the footer, simply create an html file named "footer.html" with the required footer tags and save it in your skin folder. The contents of the footer.html file will be displayed instead of the Processmaker default footer in your custom login page.