This is a background tile. You will need to upload it to your blog and set the background to tile (see your individual blog "help" on how to use a tile for a background), shut off back ground scroll, and only have the outer borders set to transparent.
Point at the picture, Click on the picture to open it in full size, then right click it and choose "Save Image As" and save it to your computer. You can then upload it as a background!