Do I need to use PNG or BMP files?
The bitmap format you use depends of course on what you want to do with it.
As a general rule it is best to use .bmp files in ProntoProEdit NG whenever you don't need transparency, because they will render faster and take up less space.
In case you need transparency you have to make a decision between the need of alpha blending or binary (bitmap) transparency.
Alpha blending using the .png format will give a nicer effect, but it will take up more space in the configuration file.
A good rule of thumb is the following:
When you are planning to use buttons on different colorful backgrounds use alpha blending, in other cases optimize the bitmap .bmp format for the main background color and use bitmap transparency.