Issue at Hand
You are unable to change your desktop background picture using the
Desktop Settings Menu. I have referenced this guide and this Manjaro forum post that appears to be related. I will be offering up three possible solutions.
Using XFCE you can always right click on the picture that you wish to become your background and select the option "Set as Wallpaper". Make sure you are navigating to the picture using your File Manager, most likely
Thunar. You do not need to navigate to the
Desktop Settings menu to do this.
Desktop Settings menu you are using should look something like this:
If so then what you want to do first is make sure that under the style drop down menu that an option such as "Centered" or "Tiled" or "Zoomed" is selected, and not the "None" option.
Once you have verified that, when you go to change what folder you are using to get backgrounds from, Select "Other", then use the File Browser to navigate to the folder you want to use, such as
/home/user/backgrounds and select "Okay". Then from the
Desktop Settings menu you should see the center pane populate with the image contents of that folder. Select the desired picture for your background from here.
Now you should have your desired picture as your background and will be able to select any picture located in the
By default, XFCE keeps wallpapers at
You could move or copy any image you wish to use as a background to these folders and then in the 'Desktop Settings' menu find your picture in the middle pane with the default backgrounds.
If none of these work, please update your post with any errors that come up. Best of Luck!