Quoting Neil Banfield,
Select the random background option in Deskscapes.
Would be awesome if we could get added the ability to choose which get randomized.
For instance, my newest DX theme is intended to be run with space dreams. It would be awesome if we could choose just space dreams to alternate with it. Just a thought.
Settings > Folders
Put in a folder path to your space dreams folder after you create one and place your space dreams in it, untick your other folders then it should work fine.
I categorize my Dreams like this mainly to have a folder of minimalistic ones for working to.