It's not currently possible, but is planned for a future version. We'll be sure to post an update here as soon as it's available.

Thanks!

We've just released DisplayFusion 6.0 Beta 7, and you can now combine monitors while using the Different Image background mode, so that you can have an image span certain monitors, but not others.

Details on how it works can be found here: http://www.displayfusion.com/KB/WallpaperCombining

Thanks!