Lol, I think at one point all of our brains have exploded... i think it's inevitable.
So let me get this straight, you're wondering if you can:
-Take an existing image and replace the layer without changing the alpha channels to accommodate new shapes/sizes.
-Keep existing properties of the default image just change the picture.
Yes you can, in theory, although I don't know how affective it would be. If you choose to keep all the same shapes, sizes then yes you definitely can. But the existing User Interface is clumsy, and poorly assembled (sorry Turbine.) Alpha channels are actually quite easy once you get the hang of them.
When a selected Alpha Channel is visible, all that is white will be seen in game, and black will not. What I like to do to achieve a simple alpha channel is:
-After I have completed the image, I save as a xxxx.psd (photoshop) as well as a xxxx.tga.
-Keeping the xxxx.psd open, I open the xxxx.tga I just saved.
-In a standard scenario I will use the magic wand to select everything I don't want to have visible in game on the xxxx.tga image. (Keep in mind that both xxxx.psd and xxxx.tga are the same image, only the xxxx.tga has been flattened to only 1 layer.)
-After the selection is made, you will need to invert the selection (top menu Select>Inverse.) You'll notice that the rest of your image is selected (what will be visible in game.)
-Now go to the top menu and click Select>Save Selection. It will open up a new window. Name the new channel you are creating 'Alpha Channel'.
-Your alpha channel is now created. It's located in the same window as your Layers although instead of being listed under the Layers tab it's listed under the Channels tab.
-So go back to xxxx.psd (the file with all your layers) and click on the channels tab. Channels tab has the info available for the RGB etc.
-Go back to xxxx.tga and drag the alpha channel you just created into the canvas (window) of xxxx.psd.
-Make sure that the alpha channel you just created is the only alpha channel in the channel list (excluding RGB, R, G, and B)
-Done, you have the alpha channel you just created in your master xxxx.psd file. You can now save your xxxx.psd as a .tga which will be used in game.
Hope this helps guy. Feel free to ask questions.