Sorry, but using Photo Gallery does not allow resizing a picture to place as desktop background.
On Windows 7, I can use Paint to resize a photo, then right-click the resized photo to use it as my desktop background. Not so with Windows 10. Maybe Paint or Photo Gallery resizes the picture, but the photo still displays on desktop background at full, original size. What's the fix? Do I have to change the desktop resolution?! Seems that shouldn't be necessary.