Quote:While I am at it, one question:
Is it very difficult (for a newcomer in GIMP not very technically minded) to replace the sky in an image by another one?
As with all graphics, it depends. A solid plain colour and you can colour pick or fuzzy select. Cut out the selection and replace with something else. That is what most beginners use, and often results in that border of pixels you found.
For solid colour backgrounds all explained here:
https://www.gimp-forum.net/Thread-Backgr...r-graphics but it is a bit technical
For more complicated backgrounds, often the Gimp Foreground select tool will make a selection of a complex shape. There is also the path tool used to draw around a subject then converted to a selection.
For non-destructive editing, use a layer mask, and either paint out the background by hand, or use a selection or channel.
When it comes to complex shapes, you will find your own favourite methods, this black-and-white created from a reduced colour copy. Used as a mask to replace a blue sky with a cloudy sky.