Post Icons?
 

How do I make post buttons the "forum_new.gif" and "forum_old.gif" for a skin? I don't want to replace them for every skin, I want a seperate new post icons for this skin.

Those aren't Post Icons those sound like "Status Icons"

In the style, all style options, look at the file path listed under:
Item Status Icon Folder

Make a copy of all images in this folder somewhere else and update the folder path to use this new location.

Now find any images you want to change and simply replace them in the new folder.