I’ve been messing around with wallpapers for the Raspberry Pi 4 and have seen that it can be flexible with bigger resolution images and smaller ones. Simplistic and minimal images work better for visibility as well as placement of the contents.
I ended up making this 1920x1080 image reference for making wallpaper. Leaving the right hand empty is usually due to the placement of boxarts & thumbnails. Keep in mind that there was talk about making the Icons bigger such as 512 & 1024 sets.
I’ve used Krita and GIMP for this, both are Free and Open-Source Software (FOSS) available for other platforms. There is also a free online Background remover tool you can use for images without transparent backgrounds.