Create an image in Photoshop that you would like to use. You can either create something yourself using the various drawing and paint tools, or you can bring an image in from a digital photo that you retouch. This image can be either something realistic, like a landscape, or just an abstract pattern you happen to like.
Save the file as a a high resolution JPG, and burn it to disk. The image itself doesn't have to be enormous or greatly detailed, since you will be using it as a template for helping you paint, rather than using the image directly as your backdrop. Of course, you do want enough detail to know what you're painting. Now take the disk to a store like Kinkos or Staples that can turn your image into a slide for a slide projector.
Take a thick large bed sheet that you don't want and secure it on a clothesline outside on a clear dry evening. Pin the bottom edges to the ground using tent pegs. Make sure that the sheet is as tight as a drum.
Run an outdoor extension cord to just in front of the sheet, and plug in your slide projector. Put the slide in and project it onto the sheet.
Using the projection as your guide, and acrylic fabric paints, take your brushes and paint in the image you see. Start by doing the outline of each shape in the image, and then work your way in. It's a bit like painting by numbers, or painting a wall mural. Try to stay to one side of the projection as you work. And when the paint dries, you have your own custom made backdrop.