you'll first have to go to a top-down perspective, then click on the TS6 menu->display options to bring up the UI.

there's a box there that says "show background image/color", right click it to bring up it's options. then click on the Blank button below where it says Background image. this will allow you to choose the image you want (I use TFG's, as BMP and JPG's get funky.) there's a size limit on it, but i can't recall what it is (just make sure both directions are same size ie 768x768, else you get soem wierd stretchings).

if the image didn't autoshow, left click the first button i told you to right click to get it to show.

you should now have a nice top down gridded view to layout and draw your item from the image.