I've probably built wallpapers for desktop and mobile by hand for years and finally decided to automate it somewhat. Just use this Sketch file (who still uses Sketch right?), place your wallpaper image on the Symbol layer and it will export nicely sized desktop and mobile wallpaper with ease. The best sized image will be 5000x5000 with the focal point of the wallpaper centered in the middle.

After you've got your Symbol layer in place, click the Wallpaper layer and you'll see 10 different sized versions ready for export. You can of course tweak each wallpaper version independently. Export all and boom, you've got your social media sharing all covered.

This sketch file uses an image of Austin from Carlos Alfonso from Unsplash https://unsplash.com/@csfoto