OS X :: New Monitor Wallpaper Changed With Wrong Alignment

Historically I have had two matched monitors. I maintained two wallpaper folders, one for the left monitor and one for the right. My wallpaper cycled randomly taking pictures from the appropriate folders for the respective side once a day. I didn't use any add-ons, it was just the basic Desktop pane. It was perfect. Recently I purchased a lovely new monitor and decommissioned one of the old ones. The new monitor is widescreen so I made a third wallpaper folder for that one and set everything up to match the way it was before. Every day when the wallpaper refreshes, both monitors now pull from the same folder. I have deleted all but the widescreen folder from the list in System Preferences for the widescreen monitor and it still somehow keeps pulling its pictures from the same place as the remaining old monitor.

If I go into System Preferences, it shows that it's pointing at the widescreen folder correctly and manually clicking random on and off again to reselect a new wallpaper will pull it from the correct place. It's just when it's automatic that it screws up. I'm not sure I've explained this well, but the result is that I have to manually set the widescreen monitor's desktop picture each time or enjoy a horribly stretched image with the wrong alignment. In the long term I will replace the other old one so this won't be a problem (I'll just delete the wretched old folders entirely), but why is OSX going bonkers in this way when it's never had the problem. I could just decide on one static image until it's fixed but I like an element of chaos in my computing, just not a stretched image.