i don't use such a background, but /help /background should let you find syntax that will do what you're already doing from the upper-left menu. You can then put the command into an event that starts each time. i.e. something like
on *:START:{ /background -mf c:\path\image }
I put a 1080x1200 jpg as background for the status window, and it only works while the status window is small. Once I drag it large enough, the picture suddenly winks out before it had a chance to display the entire image.