Looks like a mal-formed url
try: https://www.gimp.org/tutorials/CircleImage/
basically:
Layer menu -> Transparency -> Add alpha Channel
Make an elliptical (circular) selection.
Selection menu -> Invert
Edit menu -> Cut
try: https://www.gimp.org/tutorials/CircleImage/
basically:
Layer menu -> Transparency -> Add alpha Channel
Make an elliptical (circular) selection.

Selection menu -> Invert
Edit menu -> Cut