Adding a background to mac applications?

Now you have discovered one of the differences of Mac and Windows. Mac OS X is a document based multitasking OS. Windows is not document based and is designed as a single window non multitasking OS.

Because Mac OS X is a document based multitasking OS it's designed so the user has full available use of the desktop for true drag and drop between apps, this is why Photoshop and Word do not have a canvas like in Windows as this would defeat the purpose of the multitasking design. You may not find it useful now but over time you will find it better.

Now, in Photoshop CS3 you can click on the canvas pallet (it may be called something else) but it looks like 2 small squares at the bottom far left. If you click it multiple times you can set it as a full screen canvas in black or gray without changing your document size.

 

You mean you want sub-windows within a parent window ala Excel, Photoshop, PaintShop Pro, almost all IDEs, etc on Windows. It's called a "Multiple Document Interface" and looks like this:



It's a paradigm difference. Document vs Application, and I don't believe you're going to find much in the way of haxies to "fix" this. About the closest you'll come to a Apple-provided "fix" for this is probably going to be "Spaces" or some other Virtual Desktop solution.

OS 9 did have a feature to hide desktop icons when the Finder was not in view, but this has not been implemented in Mac OS X (and I doubt it ever will be). You could try things like BackDrop, and its ilk.