If you have a photo that is larger than your background resolution, choose the "fit" setting in desktop > personalization ... Background.
However if you want to edit photos, including resizing them, then you use photo editing software. I use paid software called Paint Shop Pro but there are numerous free products you can try. See the "Image Creators / Photo Editors / Viewers" section of our Freeware DB.
Note: for your needs avoid IrfanView, as it is a viewer, not for editing.
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