There’s a fairly simple registry hack you can do to make Windows look in a different folder (The standard warnings about registry editing apply here)
Change Wallpaper Folder
Open up regedit.exe using the start menu Run box, and then navigate down to the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion

On the right-hand side you should see a value called WallPaperDir, which defines the folder Windows uses to populate the list. If the key does not exist, then you can create a new string value with the same name.
Double-click to change the value, and then paste in the full path to your Folder (Note that you could specify any path here if you wanted to.)

The change should be immediate, the next time you open the Desktop panel… but you’ll notice a number of default images still in the list. Turns out that Windows also queries the Windows folder for images…
To get rid of those, browse to C:\Windows and then look for a set of Bitmap files:

You can delete these images to make them stop appearing in the wallpaper selection list, just be careful not to delete anything else in your Windows folder or you likely won’t be able to boot anymore.

There’s a fairly simple registry hack you can do to make Windows look in a different folder (The standard warnings about registry editing apply here)
Change Wallpaper Folder
Open up regedit.exe using the start menu Run box, and then navigate down to the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion

On the right-hand side you should see a value called WallPaperDir, which defines the folder Windows uses to populate the list. If the key does not exist, then you can create a new string value with the same name.
Double-click to change the value, and then paste in the full path to your Folder (Note that you could specify any path here if you wanted to.)

The change should be immediate, the next time you open the Desktop panel… but you’ll notice a number of default images still in the list. Turns out that Windows also queries the Windows folder for images…
To get rid of those, browse to C:\Windows and then look for a set of Bitmap files:

You can delete these images to make them stop appearing in the wallpaper selection list, just be careful not to delete anything else in your Windows folder or you likely won’t be able to boot anymore.