iMac - Dual Monitor Wallpaper Problem

I recently acquired a new monitor which I am using as my main one. I have set different wallpapers, one for the monitor, one for the iMac. Every time I start up the machine the wallpaper on the iMac is how I left it, but the monitor has reverted back to the default wallpaper. When I go to preferences to change it the wallpaper I chose is selected but not showing. To get it to show I have to click off it onto another wallpaper and then back on the one I want. Anyone know how I can fix this?

If you have the external monitor setup to "mirror" the iMac, that will occur each time you restart. You can try setting the external monitor to be the default to see if the wall paper will stick. You do that by dragging the menu bar from the iMac monitor to the external.

Seems to have sorted it, thanks.