Gray PNG background on hover (mouseover)

Hey there,
I’m not able to find a way to fix this with my current limited knowledge, and I was also not able to find a solution searching the forums.

I have an image with a mouseover effect, and when I rollover it the background of the mouseover png turns gray rather than remain transparent.

Can you help me get to the bottom of this issue? I’m sure it’s an easy fix, I just have no clue… have already tried Firebug but I can’t tell what I need to edit.

Thanks in advance!