Help with drawingpad add on

Hello, my problem is about the drawing pad add on. Here is the following link:

It works, when I edit a new page, but when I want to edit an existing image in a flash card, then java cannot find this image. Java gives me the following problem:

javax.imageio.IIOException: Can’t read input file!
at Source)
at com.bytehamster.drawingpad.Main.createCanvas(
at com.bytehamster.drawingpad.Main.launch(
at com.bytehamster.drawingpad.Main.main(
I do not know what to do because I have no knowledge about Java. Perhaps, it could be that I select the image that I want to edit in a wrong way. How to I “select an image” in anki while editing? I hope there is someone who can help me about this problem.