Digital BW, The Print

As an aid while I image edit, I'd like to be able to have a file of a step
wedge open on my desktop that I can lay over an image while I manipulate it,
so as to better judge where some of the tones in my image are falling. My
only beef with what I'm able to do so far is that the image is contained
inside of the typical OS window, with it's scroll arrows, et al, (made worse
in PS with it's mag view percentage and document info etc,). Is there an
application (I'm on a Mac BTW) which will allow me to have this file open
without it being bounded by the gray of a window? I want to be able to butt
the tones of the two files up against each other without some intermediary
tone of a window's border between them.

One thing I thought of was that I could create it as a PS document that I
can drag into each image as a layer, but that just seems like overkill, to
have to actually add it into every file, and worry about compatible
resolutions and file modes, etc. I'd really prefer to have it just open on
the desktop to be shoved around as needed.

So far Apple Picture Viewer has the smallest window borders, but is there
any way to eliminate them altogether?

Todd