"Joiner" is the word used by artist David Hockney to describe the photo collages he made as a response to his dissatisfaction with the medium of singular photographs.
This video is a short documentary about Hockney and his joiners.
And here you can see some of Hockney's joiners.
This app is intended to allow you to create digital joiners. It is currently a work in progress, but it does allow you to add and position images, as well as save a joiner.