And here is a text tutorial on how to actually do the particular clipping mask effect with images:
1. On your signature/canvas create a new layer
.2. Take your rectangle Marquee tool
and make a selection for the general size that you want your clipping mask to be.
That will be the general thickness of the clipping mask, but not its final orientation.
3. Fill the selection, on the new layer you previously created, with white.
4. Once filled with white, copy these layer styles (feel free to adjust them to whatever you want, there is no limitation on imagination!)

5. Once you add those effects, click ok, and then click "Control + T" or "Command + T" for OS X users to transform the object, then rotate it at the desired degree (holding shift makes it lock, and jump at certain degrees, very useful)
6. After you adjust the orientation, hide
the layer with the white shape that you just created by clicking the little eye next to it.7. Now create a new layer
and go to Image > Apply image. Click OK without altering any of the settings.8. Make sure the layer that you just click apply image on is above the layer where your white box is.
9. Now right click the layer where you applied image, and click "Create Clipping Mask."

10. Finally move around the layer which you just clicked "create clipping mask" on to get your wanted affect!
The final result of this can be found in the following signatures:

Hope this helped, comment with any critique or question!
Edited by Tramposch, 12 March 2010 - 08:01 PM.
















