Learn how to create custom desktop wallpapers in Photoshop by following a couple simple steps. Start by creating a new document in the size of the target desktop wallpaper - for instance if you are creating wallpaper for a 15" Macbook, use 1440 x 900 pixels. Next, open all the images that will become desktop wallpapers and drag them into the original sized document. Resize the images if necessary.
After the images are properly sized, go to File - Export - Layers to Files. This will make each layer its own file, allowing many images to be saved at once.
How to Make Desktop Wallpapers in Photoshop