Rounded corners in a file are different from rounded corners applied by webpage CSS. A file mask removes the corner pixels into transparency, so the shape travels with the downloaded PNG.
Rounded Image Corners uses a selected corner radius and preserves the original rectangular dimensions. The areas cut away become transparent.
Understand what the radius controls
A 400 by 300 source still downloads as 400 by 300. With a radius of 40, the rounded transition is drawn at each corner. Increasing the radius creates a more pronounced curve.
The effective radius is limited by the source's shorter side. A value larger than half that side cannot create an endlessly larger corner arc within the same rectangle.
The preview's checker pattern represents transparent space. It is not printed into the downloaded image. A receiving application that supports PNG alpha can show its own background through those corners.
Keep PNG for the mask
Converting the result to JPEG flattens its transparent corners onto a solid colour. The rectangle then has visible coloured corner areas again, even though they may blend into a matching page background.
The mask uses hard pixel edges rather than an advanced antialiased vector path. Very small thumbnails may reveal a stepped curve, so inspect the size you intend to publish.
This operation does not identify a subject or remove the background around a person. It only cuts the four corners of the rectangular file.
Save the PNG and preview it on both a light and a dark background if it will be reused in different designs. That makes unwanted opaque edge pixels easier to notice than inspecting it only on white.