All platforms: Add drop shadows, round corners, resize and do much more to multiple photos at once with Phatch, a free batch image processing program. Phatch guides a user through creating customised, reusable "action lists." Once a list is set up, it can be used to, for instance, size a folder of images down to 1024 pixels wide, round the top two corners with a five percent radius and convert them all to PNG files. Hit the "via" link below for a basic walk-through of Phatch's features.
Phatch is a free download for Linux, Windows, and Mac OS X, although each platform requires setting up the Python environment and a few other packages, which is a bit easier to do in Linux. Windows users who don't want to dive in can check out FastStone Photo Resizer, and Mac owners can try Photo Drop.