Desktop photo's keep reverting to apple one
I have my desktop picture set to a folder which contains about 50 photo's. Set to change every minute, at random order.
This works perfectly fine, as long as I use my Imac. Every minute the photo changes and another one from this folder is displayed.
However, when I shut down the Imac and open it again the next day or so, it has reverted back to a single photo from the apple folder.
I don't think apple means for me to change my settings back to my preferred ones everytime Iog in, so how do I keep my settings as I want them?