Alright, I think I know what you mean -although I'm not sure what you mean by "plaque-like" ^^"
Well, I would open that image in your art program. For example GIMP or Photoshop. Open the Layers window and create a transparent layer underneath the image, so when you use the eraser tool it shows a transparent background.
For semi transparent you would change the opacity of the eraser tool to 30%. Select a soft brush, something that looks like this:

(I'd suggest the soft brush for a "fading effect")
Change the size so it's nearly as big as half the image and work upwards towards the line.
When you save the image remember to save it as a .png image so it matches the code in the .material file.
I hope this is what you meant xD