That might be trivial – using paint or similar it is, when it comes to photoshop or gimp, it is not. Within Gimp, just select the rectangle how the border should look like. Then right click the selection, go to “Edit” and tick “Stroke Selection“. Within the opened window, select “Stroke with a paint tool” and “paintbrush“.
Then select the paintbrush you prefer (color, etc) and hit apply. Once done, save and you’re good 🙂
PS: If your gimp is german, set a different environment before starting gimp, like
$ env LANGUAGE=en gimp