Macbook Pro changing desktop picture randomly
We have seen this on occasion.  Our solution has been to:


go to Desktop &amp; ScreenSaver (Apple>System Preferences) 
Choose a different photo (or folder)
Change the "CHange Picture" time to something different.
CHange back both ("change picture" time and photo) to original settings.
close Preferences tab..


The theory we hold is that the preferences are sometimes getting corrupted, and over-writing/re-writing is fixing it....