-m is for main window...
Use -d for a single query window.
Or use -a for the active window.
If you're going to use the same image on all windows, then the easiest thing is to use:
/background -msgde Query file.ext
Just open a Query window and then use the name of that window above and then all windows should be set to the same thing by default.
Note that for chats, you may have to open one and use the -ae switch inside there.