It is very easy! It has to be posted on the web somewhere.
Be sure the dimensions of the OpenRPG map space are as big as your map will be; that's in a setting you can find in one of the menu items.
Then, click the tab in the space between the map and chat portions, I think it says General, and will be the tab farthest right. Move the radio button from "color" to "image" and type in the URL.
That's going by memory, but you should be able to figure it out from there