PNG, TGA, JPEG, GIF, BMP, ICO, DDS are supported. For translucent layers TGA is recommended. Using the TGA format, you are able to directly edit the alpha channel in your image editor of choice and avoid seams around the translucent borders of your image.
Adding an Image Layer
Drag and drop an image file into the editor. Alternative you can click on Edit -> Import Image or Add -> Image Layer.
A new dialog will open that allows you to configure the Texture Options (1.) for the import and a checkbox that will resize the wallpaper to match your image and center it (2.). Usually you want this checkbox to be enabled for your background image only, which you should import first.
After clicking OK the image will be copied into your project and compiled into a texture. It will be added as a new layer and asset automatically.