Here is one way with ImageMagick that will acomplish this without using a mask:
convert -size 200x200 xc:none -fill walter.jpg -draw "circle 100,100 100,1" circle_thumb.png
Here is one way with ImageMagick that will acomplish this without using a mask:
convert -size 200x200 xc:none -fill walter.jpg -draw "circle 100,100 100,1" circle_thumb.png