Addition to thumbnail examples...

[anthony]

Appearing in an hour or two, in Thumbnails Rounded...
http://www.imagemagick.org/Usage/thumbnails/#rounded

Circle masked Image (example)...

Using a Polar Cycle Trick we can generate a perfect anti-aliased circle mask for any sized thumbnail. Of course we will only use the distorted image as a mask for the original image, so as to get the best result.

convert thumbnail.gif -alpha set \
\( +clone -distort DePolar 0 \
-virtual-pixel HorizontalTile -background None -distort Polar 0 \) \
-compose Dst_In -composite -trim +repage circle_masked.png