PhotoScape is really good (and free) - also of those six links above, the fifth one looks really impressive (never yet used it), where it cuts out the purple flower from the green background! Here below is what the screen shows when PhotoScape is opened, and go to Editor at the top. It does many other things as well, as you see.
https://en.numista.com/catalogue/pieces71151.html
P.S. Here is a recent PhotoScape example, where I used my original pictures and made the white background so it could be added to that page, which I also created as Numista did not at the time have that coin listed ...
So it does not as such put a white background around the coin, more the other way round as it cuts the image out then it has no background, and as such is white. Same difference I guess.