I Have Some Picture's With Solid Back Ground's Can I Make Them Psd's To Change The Background? ?

I Have A Picture Me I HAve Cropped The Back Ground Out Of Just Into White There Are No Flying Specs Of Anything Or Nothing. I Want TO Change Their Back Grounds How ? I AM On A Mac What Program And what DO I Do Is This Possible Can I Convert Them What Do I Do .? When I Use A Psd The Back Ground Is Just Grey And Like Black Boxes How Come My Picture Doesnt Do that When I Import It Into Like Gimp or Photoshop ?