In this tutorial, we'll learn how to open a second image on GIMP so you can edit two files at the same time. Although this sound simple, GIMP's default behavior when you do the obvious thing of just drag and dropping a file onto it is to add the image as a layer, which isn't what we want to do. We want to open two images as separate documents.
To open a second image on GIMP, simply drag and drop it into the upper part of the toolbox. No, you didn't read that wrong. You have to drag and drop it into the upper part of the toolbox, or it won't work. I'm not joking.
Observation: on Linux Mint, Wilber's box will be highlighted in blue when you drag a file over it, indicating that it's a valid drop area. On Windows, this indication doesn't occur.