Ok so did some testing and deleting/renaming the default bezel so RocketLauncher doesn't find it leads to the emu just being imposed over whatever was in the background at the time. Might be a desirable effect depending on your theme but it didn't look good for me so I tried the transparent .png file. The effect you end up with is the emu on a black background so here's instructions on how to replicate this:
-Open GIMP and make a new file to the resolution of your screen, I have a 1080p monitor so my file would be 1920x1080.
-Press the minus key(-) to zoom out so you can see the whole canvas.
-On the right you'll see a menu for your layers and you'll see a layer called "Background." Right click it and select "Add Alpha Channel." This will add transparency info to the layer.
-Press R to pick the rectangle select tool or you can find the button for it just under the edit menu in the top left. Go into View up the top and make sure "Snap to Canvas Edges" is selected. Click and drag to select the entire canvas. Don't worry too much about being accurate, you can drag the sides to where they need to be after and it'll snap in place. Simply hover your mouse near a side and when a yellow rectangle appears you can drag it to the edge and drop it when you see it snap in place.
-With your rectangle selected(you'll see a little square in each corner if it is) right click anywhere in the rectangle and pick edit and cut. You should see a grey checkerboard pattern. This is now your transparent layer.
-Go to File and Export it to wherever you want. You should end up with a file called "Untitled.png". Rename it as needed to make it work as a Bezel for the required game and put it where it needs to go. For me at least with RocketLauncher an example file path looked like this- D:\RocketLauncher\Media\Bezels\Sony PlayStation\Point Blank 2 - and the folder had two files in it, Bezel.ini and Bezel.png, with the .png being the file I just made. Whatever backend you're using just find where the bezels are kept and sub in your new file in the appropriate place and format.
Hopefully that gets you sorted. If not I'll be happy to help some more but it's late now so it'll be a while before I reply.
Best of Luck!