Right, i'v now been able to find out how to do this. Desktop patterns gives off the impression that you can drag and drop because the pattern area gets a blue ring around it when you hover a pic over it, this isn't the way i did it.
I used an image editor (colour mac cheese) and the select tool to copy the selection to the clipboard. Then in the edit menu of Desktop Patterns i selected paste. You might need to raise the memory allocation of DP to get it to play nice with larger images