<body background="/yourtilesname.gif">
Whatever image you put into the background field will "rerun" so to speak to cover the entire background area of the site. You can do this with any image, even a photograph if you want to test it out before making a tile.
You can make the background tile any size you want depending on how much space you want between logos. On your example page you listed, their background tile is 96x96.
One word of caution... make sure this image isn't large or heavy in KB or it could slow down the loading of your page. I would suggest making the tile and then crushing the image so that it is as low in KB as possible to prevent this.
Hope this made sense. I explained it as best I could, but I am not a web designer, so hopefully the terms I used make sense.
