Here is a possible situation. You have two monitors, your main one on a higher resolution than your second one, e.g. on your first monitor you have 2560×1440 resolution, and 1600×1200 on the second monitor.

Your wallpapers look fine in your main (higher resolution) monitor, but in the second they are distorted, with objects becoming ‘fat’ — shorter and wider than they should be.

To overcome this problem open the “personalization” properties and go to the Background section. When you are there change the position of the wallpaper to “Tile”.

On the main (high res) monitor you wallpapers will keep appearing as always, and on your second monitor the distortion will disappear but you will be able to see an even smaller area of the wallpapers, the rest being invisible, i.e. you will see only 1600 of the 2560 width, and only 1200 of the 1440 height.