Press F5 when in preview, then the stylechanger will make that the image.

When in preview, you mean the editor screen or when I'm testing the chamber?

When testing the chamber.

Okay, it changed the image, but it's not the image I want; it's instead the image when you first enter the chamber.

Press F5 while playing to take a new screenshot. (Nothing will appear to happen, don't worry.)

I keep doing that & it doesn't work; it just takes a snapshot of the beginning of the level.

steamapps/common/portal 2/portal2/screenshots. Find the image. Open with Photoshop. Put a layer over it with anything you want, such as a cube. Add some fancy shmancy text. Merge that layer. Click on save.

Pushing F5 no longer works. (And holding Ctrl no longer loads the map from the elevator). :(

Have you run the stylechanger exe? The vbsp overrides are used to allow the Ctrl-elevator and the screenshots to work.

Check this guide http://steamcommunity.com/sharedfiles/filedetails/?id=244711878