Hello JJ.Lin,
The easiest way is to create a png image fully transparent using your favorite software like paint, paint.NET, photoshop, gimp, etc. and use this image as the background.
The background is mandatory because this is what sets the dimensions of your progress indicator.
Otherwise, you can create your own widget based on the image progress widget but you will not be able to set it in the Designer and everything will have to be done by code.
Hope it helps,
When your question is answered, please close this topic by choosing Select as Best.
/Alexandre
