I like to delete my previous profile picture. How can I delete it? What about images uploaded to i.stack.imgur.com?
When you upload a picture to use as a (Gra)vatar, it gets uploaded to SE's Imgur account. In that sense, it cannot be deleted.
As an approximation, if you upload a different image (even if you do not use it), then that image will become your option for an "uploaded picture" (pushing your previous uploaded picture into the æther).