Taking a look at the built in /Library/Desktop Pictures/ directory and contents as an example, those image files seem to be set to the following owner, group and permissions
owner: system read & write
group: wheel read only
other: everyone read only
I would say then that, since we can obviously use those images as desktop wallpaper, using those same permissions should be fine, meaning set the owner to "root" or system and the group to wheel and set the read/write permissions as above.
I would test it locally, placing the image file in its final location on your admin Mac and setting those permissions and see if you can choose it as a desktop image. It should work.
