.mph files are image template files for MonkeyPhoto. MonkeyPhoto is a discontinued application for photo management, greeting cards, etc. It was developed by a company named Monkeymen.
All the MPH files we've examined are just JPEG files with an unconventional filename extension. So, MPH probably isn't actually a format. We document it because it appears in some lists of graphics formats.
More research would be needed to determine if MPH files can have any formats other than JPEG.