How to disable flashing image in a button?

I have a Button which is linked to an ImageList. I change the ImageIndex over a OnClickEvent.

Now i got the problem that when i change the image with the OnClickEvent it changes, but if i move my mouse over it it shows me the old one and is pulsing.

Anyone knows how to disable this pulsing / flashing / swapping?

EDIT: HotImage were Index 0.