My avatar looks distorted-question about dimensions

Hi there-

I changed my avatar yesterday. The file was too big so I resized it using paint.  OH's site said it needs to be 100 x 100 pixels. Yet when I resize it-it's not that because the picture is not a perfect square. Yet when I upload it-I think it is making it 100 x 100-which makes it look really weird.  It looks normal if you just see it in the forum but if you roll over it and the big picture pops up-it's all distorted.  How can I fix this?  I'm not very tech savvy!

Thank you in advance for your help!

Pam