Change toolbar button image - where or how

Want to change an image for a toolbar button. Found the file(s) to change, I thought.

I have changed these 4 files:

editor/plugins/icons.png

editor/plugins/icons_hidpi.png

editor/skins/moonocolor/icons.png

editor/skins/moonocolor/icons_hidpi.png

When I browse to these 4 images, the browser shows the updated png. But in the editor, the old image is still displayed.

What am I doing wrong?

thanks for the great editor!