The Wall-Paper tool allows setting an image from your collection as a background photo on your desktop. If you use a multi-screen computer, the image is assigned to the current monitor where digiKam is running.
Select one image that you want to push on your desktop and go to, or use the icon Set as wallpaper from the Tools tab on the right sidebar. A dialog will appear to customize the Wallpaper Layout of the background photo. The possible choices are listed below:
Adjusted: The image is resized to the desktop size without preserving the aspect ratio (Linux and Windows).
Adjusted with Aspect ratio: The image is resized to the desktop size preserving the aspect ratio (Linux only).
Adjusted and cropped: The image is resized to the desktop size and cropped to support the desktop aspect ratio (Linux only).
Mosaic: The image is displayed multi-time as a mosaic to fill the desktop (Linux and Windows).
Centered: The image is centered on the desktop without modification (Linux and Windows).
This dialog does not exist under macOS. The layout is always set as Centered.