I cropped a rectangular image into a round image, but when I try to save it puts a black border around where the original picture was and I'm unable to save as JPEG when R clicking on the cropped image.
Lunapic.com lets you quickly crop an image to a circle!
The circle is made by adding a transparent area around the image (the image itself will still be a square or rectangle)
JPG file formats do NOT support transparency, nor circular images, so your image is converted to PNG to make it look circular.
It must stay in that format to best preserve transparency!