One way to cut the corners of image EQUALLY.
Turn on a grid which divides evenly into the dimensions of the image
Turn on snap to border and snap to grid.

Use the polygonal lasso tool to select the area to be clipped. Set the
tool to the "add to selection" mode so that all 4 corners can be selected.
The tool will snap to the grid lines to easily make the four corners equal.
After all four corners are selected, press the backspace key to delete the corners.