\"the clipboard\" is a temporary storage space. You don\'t see it. When you \"cut\" or \"copy\" something and then \"paste\" it somewhere else, it is stored in \"the clipboard\" temporarily.
To make a front/back montage in Gimp...
1. load the front image
2. \"select\" the portion you want to show
3. use \"scale image\" to make it the size you want
4. repeat for the back image
5. create a new image that is as tall as the front and back images put together, and as wide as the wider of the two.
6. \"select\" and \"copy\" the front image and then \"paste into\" the new image.
7. \"select\" and \"copy\" the back image and then \"paste into\" the new image.