To change a player image, you need to replace the original graphic file in the publishing project with a graphic of the same name.
If you are replacing an image in the actionbar.png file, you will need to size the image to the same size it occupies on the bar.
In this example, you will change the graphic used for the Preferences button in the Player. The Preferences button is part of the actionbar image and is sized as 16 x 16 pixels.