If I remember correctly, don't newly added actions that display in center fade away after a while?
If that's true, you could add some code that waits the length of time it takes the center action to fade out, and then remove that action, and re-add it without the image, and showWindow set to false. This is what the biki says about the showWindow parameter for the addaction command:
showWindow: Boolean - If set True; players see "Titletext". At mid-lower screen, as they approach the object. False turns it off.
It might work (even if center actions don't fade on their own, deleting the action after a wait should still remove it from center screen).
Hmmmm...I guess this wouldn't really solve the problem of the image being in the action list for awhile though...