What you can do is use a pixmap as an icon and then put this icon onto the button.
To make sure the size of the button will be correct, you have to reisze the icon according to the pixmap size.
Something like this should work :
QPixmap pixmap("image_path");
QIcon ButtonIcon(pixmap);
button->setIcon(ButtonIcon);
button->setIconSize(pixmap.rect().size());
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