i think the OP means this icon (click "customize panel" in "albums" view, tick "show click to play button..."):

i remember that this icon has been initially included in localization files (as a localization string entry using base64-encoded image; because it was a localized text "Play" at that time, not the icon), but MB doesn't use that localization entry anymore.
and from my screenshot (it's hiccup's "nocturne" skin) it seems that it can be overridden by bitmap skin.