Hello Dieter,
To be able to put the changed images back to your skin you should load the same skin before that.
Notice, that the first part of the images name is the name of the skin that they belong.
No matter which approach you are going to use you should also consider that each skin element along with the image also has properties like Margins, Padding and SizeMode that change the appearance of the image applied on the controls.
These properties can be modified in the skin designer in the State Properties panel.
Regards,
Angel.