[Judobreaker]

That's pretty tough.
To do this you will at least need a few expandable images in your design.
Like the forum, the right part of the header is the expandable part, it is the part of the banner getting bigger and smaller without getting ugly.
The footer of these forums also have an expandable part like that.
The rest is simply the text areas expanding.

In the case of the layout you just posted, don't even try it, there are no expanding parts and especially the background on the content part makes it extremely hard.

Creating expandable layouts is almost like an art itself.
Of course there's easy ones, but once you want to be a little more original it could get pretty hard, the hardest part being the getting it to work in every browser... :P
But with some practice you'll get the hang of it I guess.

Anyways, what you should start with is creating a layout that is suitable for expansion.
Once you have a working layout that is suitable for expansion you will have to edit the html code.
In an avarage layout made with PS you'll have to set the table width, and also all the width of the expansion parts and the text area, to 100%.

Now if you do that the layout will look quite weird.
To fix that the last width you will have to set to 100% is one of the spacer <td>'s at the bottom of your table.
Which one you need differs per layout, but in simple layouts it is usually the largest one.