How to Add Stroke / Borders on Selection
How to add stroke lines or borders to an image selection in GIMP?
Explanation
Stroke Selection is an act of striking the selected portion of an image.
Steps to Follow:- Open GIMP.
- Click File -> Open and then choose a file. Click Ok button.

- Now select a portion of the image using selection tool.
- Click
Edit->Stroke Selection or else you can access it through clicking
Select->Selection Editor.
- Then a box named
Stroke Selection appears.

Normally, you can strike the image either by applying
Stroke Line or with the
Paint Tool option.
Stroke line:- Click the radio button named Stroke line and then set the Line width.

- Pick either Solid color/Pattern as you require.
- When you click the
Line Style, a few more options appear. Set the "Cap style, Join style, Miter limit, Dash pattern and Dash preset" as per your need and finally choose Antialiasing option if you want.
- The
Antialiasing option smoothens the curved strokes or strokes drawn over an image.
Use this option to soften the strokes drawn over an image.

- Finally, the click
Stroke button.
- Wait until the image gets processed and then you can find the stroke on image.
(View the below image)

* To gather more information related to stroke selection, just use this
Stroke an image with Paint Tool link.