The easiest way to do what you want is to make sure the images for each Skin has it's images in a separate subfolder. Duplicate your existing skin. Make sure the new skin is set visible, then select all the attachments in the skin, go to "Find And Replace" and then change the path of your attachments.
For example say you have a skin called "Red". You duplicate this, then rename the skin "Blue". Now in "Find and Replace" search for "Red" and change it to "Blue".
I hope this helps.