I have had this same issue with OS X Lion and it continued on to OS X Mountain Lion as well. I have had the wallpaper switching on me to the default solid blue color whenever I restarted (every month or so) and when switching between users (every other day or so). So this issue was very frustrating having to go back in and reset the wallpaper to my preference whenever I switched to either user. I found a solution that has been working for me for several restarts and several switching of users now:
First, I took some of your advice by locating all of my wallpapers into a 'Wallpaper' folder located in each user's 'Documents' directory. I copied to same wallpapers to each user so both user's have the same wallpapers to choose from.
Then, I installed the latest stable version of 'OnyX' (freeware software). I used OnyX on both user accounts to run through practically all the settings to clean up the entire Mac (Mac Mini, mid 2011) including: Verify [S.M.A.R.T. Status, Structure of volume, and Preferences], Maintenance [Permissions], Cleaning [System cache, User cache, Internet cache, Fonts cache, Log cache, Misc. cache]. I generally left the default checked boxes alone and didn't change any settings and Executed the program to run as default.
After running OnyX, I re-set my wallpapers for each user and went ahead and tested the restarting of the computer and switching back and forth between users. It was very easy to duplicate the bug previously, but now I could not get the wallpaper to switch back to the default blue as before. Everything seems stable with my wallpaper selection now.
Hope this helps someone. Thanks everyone for your input too.
As a side note, my Mac is running much smoother and quicker now too. =)
I have read through all the posts on this topic. It's all incoprehensible, ineffective for me.
With Mountain Lion, I can have as many desktops as I like and can set a different background picture on each, no problems. I can also choose a folder of pictures to change the background randomly, as long as I choose the SAME set of pictures for every desktop. But as soon as I select a DIFFERENT set of pictures to change randomly on each desktop, that's when they all revert to the galaxy background when I select them. Took a while to figure this out, but that's it. BTW, none of the discussion about guests, logging out and in, etc. applies since it's my computer, I'm the only user and the only time I shut down is (a) when I'm going to be away for an extended period or, (b) when the computer freezes and there is no other option. Guess it's just a bug I'll have to live with.
I found the solution to this problem. Every before weren't working with my MBP Retina 15". The easiest way is just to change format from ( I think most popular ) JPG to PNG. Then you can copy this file into Desktop pictures in the library. When the wallpaper is set everything is ok after rebooting or changing users. Hope I helped You
I've been really annoyed by this and the only way of changing it is to select a wallpaper just before you update the system and it will stick, until you delete/move it from the location.
But today I was looking at good images online and chose to set a picture as my desktop background from Safari, I then restarted my Macbook Pro and it stuck, so you can always upload your pictures online, or find ones that you like.
Hope I helped.
Found this in Yahoo and it works for me, just do everything he sais, hope it work for everybody.Not "OS X". That's what all Macs since 2003 use, so it doesn't tell anything. The "X" means Roman numeral ten. Complete the version as OS 10.5, or 10.6, or 10.7, or 10.8.
-- Open Disk Utility.
-- Select the "Macintosh HD" at the left frame of the Disk Utility window.
-- Select "First Aid" tab.
-- Click the "Repair Disk Permissions" button.
-- When it finishes, quit Disk Utility.
-- Go to the top menu bar, Click the "Go" menu and choose "Go to Folder".
-- Type (or copy here and paste)...
-- Find this file and trash it:
Lion 10.7.5 joined to AD with Management Prefrences applied to computer-lists OR users
All MCX that I tried so far work OK, apart from Desktop Picture
The correct MCX gets applied to the machine/user as reflected by entry in com.apple.desktop.plist
Yet the background is always Andromeda Galaxy.jpg
As the preference is set to Often user can change background, but on next boot it reverts back to Andromeda Galaxy.jpg
That is simply insane! Tried each & every "fix", none makes any difference at all