Round Image Corners
Processed entirely in your browser. Nothing is uploaded.
A share of the shorter side, so it looks the same at any size.
Result
JPEG has no alpha channel, so a transparent corner cannot survive it — choosing JPEG forces the solid background, and the tool says so rather than silently filling the corners with black. The circle option crops to the largest circle that fits, taken from the centre; a portrait photo will lose its top and bottom. The mask is drawn once and composited, so the edge is anti-aliased rather than stepped.