Fixed TYPO "Faled"
First, you do not need to switch to another wallpaper before deleting the existing one. DMT will create a new image for showing as wallpaper as it may need to stretch/truncate the existing image to fit the monitor(s), change the image format and may need to merge multiple images together if you are displaying a different image on each monitor. This means that as soon as the wallpaper is shown, the existing image(s) on disk are no longer needed, so would be safe to delete. As for your suggestion,...
Do you get this error message every time the wallpaper changes or just occasionally or whenever something else is happening, like when a monitor resolution is changed or monitors plugged in or removed?
In the directory where you installed DMT (usually C:\Program Files(x86)\Dual Monitor Tools) there is a DmtFileLocations.xml which controls where files are written to. (If there isn't a DmtFileLocations.xml file, there will be a DmtFileLocations-sample.xml. This can be renamed/copied to DmtFileLocations.xml.) In this file, you can edit the "datadirectory" value from the default value of "%APPDATA%/DMT" to a writeable folder, and this will be used to save any settings that DMT uses along with the wallpaper...
In the directory where you installed DMT (usually C:\Program Files(x86)\Dual Monitor Tools) there is a DmtFileLocations-sample.xml You will need to copy or rename this file to DmtFileLocations.xml. In this file, you can edit the "datadirectory" value from the default value of "%APPDATA%/DMT" to a writeable folder, and this will be used to save any settings that DMT uses along with the wallpaper image. If you don't want to lose your current settings, you will need to: Stop DMT Edit the DMTFileLocations.xml...
Primary monitor group can now be specified using monitor names
Monitor names can now be viewed/edited in the monitors grid panel
Tidying up of monitor name ordering