I have not been able to find way to place a bug report, so I write here, sorry if I missed some obvious.
But It seems that if you change one of the skin images, the new image will not show up when you preview your forum. It seems weird that it adds a new version, but that the version have no effect when previewing.
It should either not add a new version at all, or show images based on version.
Do you have Studio Preview enabled on your community? If so, you should click the revision number that is in the "Preview" banner (in yellow) to vie your community in preview mode (or add /?version =preview to your URL) and then navigate to the area where this image should be replaced. Also make sure that the skin you are editing images for is applied to the community (though Admin) or else you won't see the change.
If this is a new skin you;re creating that has not been used on the community yet, be sure to deploy your new skin and any change you have made to from Studio first, and then apply it to your community though Admin.
Another potential solution:
Perhaps you have replaced an image that is state dependent (like a 'moved board' icon or 'board with a new reply' icon that is only used when a board is in a certain state... ). It would help if you could let me know the title of the image that you replaced as shown in the Studio skin images list.
If you still have this problem, please file a case on the Support Tab in the Lithosphere