Transparent Button Background On Mouse Enter

I have created a circle button with transparent corners, I set the properties to backColor as transparent, FlatStyle is Flat and I have selected my background image. Now, I have coded the MouseEnter event to change background image, which works fine, except it gets a plain white background behind it! I have tried everything to stop this, including setting the same properties as the original image into the code:

Which doesn't work! Does anyone have any idea how to get the background image transparent when the mouse hovers over the button?

Thanks,
Tom.