How crop photo with a circle

[Nisko]

Thank you for the very clear instructions. I think I understand most of what you explained - except for the transparency. I want to place the finished product on a white background (but it could be any color). What do I lose or give up by not being able to make it a jpg file? In other words, how is a png file different from a jpg file? Is it possible that the app I want to place the finished product in doesn't support a png file? Thank you.

PS: I haven't tried your instructions yet - but hope to sometime today or tomorrow (physical disability).