Do you know of a way to set all user desktop backgrounds to a standard appearance?

Yes; you can configure a Group Policy Object (GPO) that gets applied to the desired user accounts to do this. In Group Policy Editor (GPE), navigate to User Configuration\Windows Settings\Administrative Templates\Desktop\ Active Desktop and enable Active Desktop Wallpaper. In the Properties dialog box of this policy, you specify the full pathname of a .jpg, .bmp, or .html file and select whether to center, stretch, or tile the image, as Figure 2 shows. For this policy to work, you must also enable Enable Active Desktop, which you'll find in the same policy folder.