I want an image of size 4551x8001 px to turn into 16:9 format, i thought scaling would work, but it just stretches the image. How can achieve this? Changing the size leads to the gray boxes alternating, but i can overlay a black background behind it. This is the image, i would just like to turn it into 16:9 format without losing quality.(So 14224x8001)
There are some problems
That size image would be far too big. Massive in fact. Don't create such large images for on-screen use. It's absolutely unnecessary. Find the size (in pixels) of the screen/device the wallpaper is going to be used on. That should be the size you need. Raster images always look best at the native size and resolution of the device they are being viewed on.
The image is probably the wrong aspect ratio. You'd need to crop it to the correct aspect ratio first, then scale if necessary, while maintaining the aspect ratio, or the image will be distorted.
You can't rescale (resample) raster images without some degradation in quality. You can't get more detail in an image by scaling it up, and you will lose detail if scaling down.
In GIMP the crop tool has a fixed aspect ratio option. You can type in 16:9 in there to crop it that way.
Do Image > Scale Image to resize an image if absolutely necessary. Don't uncheck the aspect ratio locks, and the aspect ratio will be maintained.