This behavior round the 4 corner of the images after the upload and the resize process are completed.
The upload process, after is completed, verify if this behavior is in the page and if the behavior is find round the 4 corner of the uploaded images.
If one or more image angle are void the behavior skip the image and don't round the corner of this image. So you can define if round the corner of all the images or if skip one of them.
The corner you must insert is the top-left corner image, the behavior will turn the image to create the other 3 corner top-right / bottom-right / bottom-left.
The corner image must be on the site, is not uploaded during the upload process. You must put the corner image in the site.