Design/draw Wallpaper from existing pattern

For printing a self designed wallpaper at www.betterwalls.de it is necessary to create a file (jpg, tiff, png) with 1200 dpi resolution. The pattern and look of the wallpaper exists as a normal photo (2 colors, easy pattern, not complicated). Therefore the pattern has to be re created and saved in a file. Color of the wallpaper could he slightly adjusted.