1) Elliptical Marquee tool - select the "Style: -> Fixed Size."
2) Define the Size of the fixed Ellipse - Width & Height.
3) Use the Elliptical Marques with the fixed size to draw four circle selections a the four corners - with each circle to sides leaning against the two corner sides. Ensure Merge multiple selection with the Shift Key.
4) Once done selection of 4 circles at the corners - use the Rectangular Marquee to select two rectangular areas to select the inner region of the rectangle. Ensure Merge multiple selection with the Shift Key.
5) Once the wanted are slection is done, do an INVERSE selection and delete the unwanted corners.
You can do it in lightroom as well, very quick and easy.
In Develop module, Under Vignette > Post-Crop
set Amount = +100
Midpoint = Leave at 50 , unless you want a white frame then decrease the value.
set Feather = 0
set Roundness slider almost to all the way left, say = -90