As in the previous versions of blender you could use background images as references when modelling, you can still do this in blender 2.5 but because of the new interface some many find it difficult to navigate. I will show you in this quick tutorial how to do this.
Open blender 2.5 to the default scene. Now select view then properties or press N, this will bring up a new menu.
In the new window, scroll down and you will see 'background images', select this to drop the menu down.
Now you have more options, tick the background images box, add an image then open yours.
I have used the blender logo image for an example as you can see below. I have gone into front view pressing number pad 1 and view orthographic pressing number pad 5.
That's it for the tutorial once you can do this you will be able to use the images as references when creating models, I will show you how to split and join the windows for an easy set up in a future tutorial.