First things first, there is actually nothing as a circular image. Images that appear circular are due to the fact that the corners are made transparent and shaped such that a circle is portrayed.
With that clarification, I believe you have a clue about what you will do.
So you can go about it as follows:
First, make sure that your layer has an “alpha channel”.
Layer → Transparency → Add alpha channel
If it’s greyed out it means you already have one
- Create a circular selection with the “Ellipse select tool” (the 2nd one in the toolbox).
Use the “Tool options” dialog
Windows → Dockable dialogs → Tool options
a) If you want a true circle, use the Fixed option: select Aspect ratio and enter 1:1.
b) Depending on what kind of marks you have, you can use:
i) The diagonal framing (default): click on one corner, drag across a full diagonal and release at the opposite corner.
ii) The radial framing (check Expand from center in the Tool options): click on the center, drag across a half diagonal release on a corner.
If the selection isn’t perfect on the first try, you can move it (click around the middle) or extend it (click inside, near a border or a corner).
Once you have the required selection, invert the selection (Select → Invert, or Ctrl-I) so that everything is selected, except your circle.
Erase the selection (Edit → Clear or [Delete] key). You should have your central circle left, surrounded by a checkerboard pattern. (this checkerboard is not part of the image, it just indicates the transparent parts of the image).
You can reduce the checkerboard to the minimum by auto-cropping the image (Image → Autocrop image)
Last, save the image in a format that supports transparency, like PNG (JPEG doesn’t support transparent images…)
If you are going to work further on the picture, save it as XCF (Gimp native format).
You may equally visit this link for more clarification: https://www.gimp.org/tutorials/CircleImage/