You can resize your pic on this site:
http://www.shrinkpictures.com/create-avatar/
The avatar size for HC is 80 pixels.
Once it's resized, go to control panel then profile then edit avatar and paste the code into either the upload from an URL box or the Link Off-site box. I think it's the Link Off Site box, but I'm not sure so you could try both.
Hopefully this should work!