If you use Masque to skin your icons, HeyBarbaruiva created an awesome Masque skin to handle vertically cropped icons http://www.wowinterface.com/download...baruivaUI.html I use this for my faux "actionbars" in WeakAuras to great success.
If not, AddOnSkins also allows you to do this, I believe. I don't personally use it, but I'm fairly certain it allows you to handle horizontal stretch appropriately also.