Why do the wallpapers I put in the 'Pictures folder' in the appearence settings never save?

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http://askubuntu.com – I was in the appearance settings and I added some of my own custom wallpapers. It was all good, until I went back later only to find that all of the images I have imported where gone. Ever time I added them, they would disappear upon reopening of the appearance settings. Is there any way I can fix this? (HowTos)