I think this is the question, but I could be wrong
You can resize part of a canvas (your photo) by selection and then scaling. The unified transform is a good tool for this, hold the ctrl-key down and drag the big corner squares. Make sure you have an alpha channel Layer -> Transparency -> Add Alpha.
little demo animation of that: https://i.imgur.com/sdPzDVY.mp4 shrinking grumpy cat.
Then it all depends on the original image because that leaves a transparent hole to fill in
and how that is done depends on the image, could be using a plugin or maybe the clone tool or whatever is appropriate.
It would help if you could post an example, if it is large then try imgur.com
Edit: Just see your second post. A black background, no problem, bucket fill the under layer.
Quote:... I would like to know how to resize an image within a photo file without reducing the whole photo...snip...I want to shrink an image to make a more usable wallpaper...
You can resize part of a canvas (your photo) by selection and then scaling. The unified transform is a good tool for this, hold the ctrl-key down and drag the big corner squares. Make sure you have an alpha channel Layer -> Transparency -> Add Alpha.
little demo animation of that: https://i.imgur.com/sdPzDVY.mp4 shrinking grumpy cat.
Then it all depends on the original image because that leaves a transparent hole to fill in
and how that is done depends on the image, could be using a plugin or maybe the clone tool or whatever is appropriate.
It would help if you could post an example, if it is large then try imgur.com
Edit: Just see your second post. A black background, no problem, bucket fill the under layer.