1) What painting? Do you mean one (or some) of the old chistianlov skintone changer paintings?
Those are obsolete hacks, and may or may not perform as expected (or at all) depending on which EPs/SPs you may have installed.
2) You need to have showerproof versions of the skintones installed.
3) Rather than the older and unsupported christioanlov paintings (if that is what you are using), suggest christianlov's replacement for all of those, here:
http://www.modthesims.info/showthread.php?t=284572It is a bit of work to set up and configure access to skintones, but that's just the way it is.
Regardless, if you do not have the showerpoof version of any particular skintone, then you will get the 'floating head' effect when the Sim is unclothed.
Note: Using any of the above (the obsolete paintings or the newer, shinier tool) makes the change cosmetic but not genetic (will not be passed along to offspring).
To make a skintone change genetic cannot readily be done in-game, but can be done outside of the game using a 3rd party tool, such as SimPE.