The technique is the same as shown in Click image for larger version. 

Name:	Ink Blots to islands using Wilbur.pdf 
Views:	78 
Size:	1.54 MB 
ID:	46589 except that you just load the cloud image rather than using coast and mountain masks. You can mask a coast mask from the cloud image directly in Wilbur by using Select>>From Terrain>>Height Range and then saving the selection.