Hmm which file did you edit to change the image on the bottom right of the battle interface?

Anyways, to switch channels, first select channels (green), then make the alpha channel (blue) visible, then make the RGB channels (red) invisible.


This will produce the following:


You can see that the image is now white where anything is visibile in-game and black where stuff is invisible.
To edit it, you'll also need to select the alpha channel (by clicking on it in the channels tab).