The question is in the area of interaction of the mouse and buttons.
For example, if the picture button is triangular, but the interface is still the square.
Sorry for possible mistakes in English language.
Thanks in advance.
You can create any sort of shape you want inside an image editing program. For a triangle, just create a triangle, and make the background transparent. Save the image with the transparent alpha channel and import. In the button widget, go to appearance->style, and change the image in any given button state (normal, hovered, etc). Change ‘draw as’ to image.
