Online tool: http://www.quickpicturetools.com/en/rounded_corners/
Open the online tool: http://www.quickpicturetools.com/en/rounded_corners/. Click "Load an image from your computer." to add the image you want to process.
Click 1 in the picture to modify it to 50, which is a full circle, otherwise it is a rectangle with rounded corners. Click 2 to modify it to png, otherwise it cannot be set to transparent, click 3 and select transparent.
Click "Generate image." in the lower right corner