Set mouseEnabled = false for the transparent image control.
It might also be possible to change the hitArea for the larger image using the Sprite drawing methods.
In general, Flex is designed to have the z-order set by how low it is in the mxml (the lower it is the closer to the user it is).
It may also be possible to directly draw the image to the screen using low level methods or possibly Degrapha so that it doesn't have a UIComponent/z-order attached to it.
It should be possible to draw a transparent button on top of the large image, that receives the mouse clicks instead. Try Alpha = 0.001 if it doesn't work with alpha = 0.
Ultimately you could write your own mouse handler and use the hitAreaTest methods, etc. to see which control you need to pass to.
Well there are two possible ways to approch this problem: The first one is to check each mouse event for a hit with the current mouseX and mouseY position: To example
Another ways is to set a sprite as a custom hitarea to the object (via the "hitarea" property). If one is too lazy to draw a custom sprite for every PNG, there's a nice method that does the job automatically: example
I hope that helps.