Firstly, upload your picture on some image hosting website such as,
http://www.tinypic.com

Once done, you should get a url for that image you just uploaded. Copy that.
Now go to your steemit page settings as shown below,

http://www.steemit.com/@sabbir0174/settings

Paste the url you just copied on the "PROFILE PICTURE URL" box. Click "Update".

You are done!