Just to add, if you have GIMP,
you can simply do it by using the Fuzzy Select Tool and clicking the area around your character,
and then erasing it with the Eraser c:
(https://i.imgur.com/8Rpa49c.png)
But make sure that the photo has transparency.
You can do this by simply clicking on your photo layer and right click on it and pick Add Alpha Channel
(https://i.imgur.com/9B60r5V.png)