Probably more than a crop required, and also some unknowns.
1. Do you know the size of your computer screen in pixels ? You can get that from the computer/ monitor specification
or
Take a screenshot, open in Gimp the screensize is shown in the top bar: https://i.imgur.com/JAE4Ya2.jpg
2. You can set the crop tool to a fixed size and move the tool around for a best crop: https://i.imgur.com/yUKW1B1.jpg
3. Chances are the image is either too large or too small. Scale the image so that one side is an exact fit and the other is larger than required: So this one, https://i.imgur.com/CO5zONW.jpg The target is 1920x1080 and I scale to 1920x1435 pix
4. Now I can make a best possible crop from the image: https://i.imgur.com/CpEcDt5.jpg
5. PreView ? What will it look like as a desktop background ?
Make sure the image scale is set to 100% View -> Zoom -> 100%
The TAB key hides the docks
F11 (View -> Full Screen) does just that https://i.imgur.com/F2ycohe.jpg Still a bit of window decoration but close enough.
Back out with F11 then TAB
Edit: I might have gone over the top with that description
In Gimp the crop tool has "Delete cropped pixels" off by default. Enable that.
Instead of Fixed Size set the crop tool to Fixed Aspect Ratio. Same setting as fixed size (say 1920x1080) but now the crop area is adjustable.
1. Do you know the size of your computer screen in pixels ? You can get that from the computer/ monitor specification
or
Take a screenshot, open in Gimp the screensize is shown in the top bar: https://i.imgur.com/JAE4Ya2.jpg
2. You can set the crop tool to a fixed size and move the tool around for a best crop: https://i.imgur.com/yUKW1B1.jpg
3. Chances are the image is either too large or too small. Scale the image so that one side is an exact fit and the other is larger than required: So this one, https://i.imgur.com/CO5zONW.jpg The target is 1920x1080 and I scale to 1920x1435 pix
4. Now I can make a best possible crop from the image: https://i.imgur.com/CpEcDt5.jpg
5. PreView ? What will it look like as a desktop background ?
Make sure the image scale is set to 100% View -> Zoom -> 100%
The TAB key hides the docks
F11 (View -> Full Screen) does just that https://i.imgur.com/F2ycohe.jpg Still a bit of window decoration but close enough.
Back out with F11 then TAB
Edit: I might have gone over the top with that description
In Gimp the crop tool has "Delete cropped pixels" off by default. Enable that.
Instead of Fixed Size set the crop tool to Fixed Aspect Ratio. Same setting as fixed size (say 1920x1080) but now the crop area is adjustable.