Image cache is one of the most powerful module.You can just do anything from this module for example: watermarks,scale,crop,overlay and many others.
Here Iam going to tell how to use image cache.
You simply need to install a module of image cache and enable them,after enabling the module create a image preset and add a action named round corners in that and specify the radius suppose 16px,17px or what ever it suits you.
Save preset and refresh to have output