How to remove the black background in the image button because when we are in the on-prem version this image has no black background and it is a PNG file. But when I create a image button on O365 it has black background even if the image doesn't have.
basically any button on NF365 inherits from a CSS class where the background-color is set.
On your button, add a css class, and just put "background:none !important;", like :
and you're done!