I purchased PictureMe and it does not work. How do I re-install it?

Drag the app's icon from the Apps folder to the Trash and empty the Trash. Then redownload the app from the Purchased list in the App Store app.

ASC - Late 2012 Mac mini, macOS 10.13.3;  Watch, 38 mm AL, watchOS 4.2.2; iPad Air 2 & iPhone 7+, iOS 11.2.5; TV 4th Gen, tvOS 10.2.5;  Airport Express.