For the past few years, I have used Picturesque whenever I needed to apply rounded corners to batches of images for one of my web design clients. Picturesque did the job, but its lack of Automator support started getting to me. I had to resort to the 'Watch Me Do' action just to apply a preset to an image in a larger image processing workflow.
So recently, I took a day to write a 'Round Images' Automator action. Given a set corner radius, border width and border color, the action rounds any or all of the four corners of the images passed into it, as shown below.
The Round Images action is freely available on GitHub, so check it out if you think it will be useful in your workflow.