Rounded corners and transparency
Rounding a corner means removing the pixels outside the curve. Something has to take their place, and in an image file that is either transparency or a colour — there is no third option. This tool defaults to transparency and writes a PNG, so the corners take on whatever is behind them when placed in a document or page.
If the image will always sit on a known background, filling the corners with that colour is the better choice: the file can stay a JPEG, which is usually much smaller than a PNG for photographic content.