avatar resizing assistance please

I know this has been answered many times but how do you resize a picture to make it your avatar.  I want to finally download a picture from my "big" computer into the avatar spot.  It says it is too large but I didn't pay attention to how you resize them.  I know they should be 100 X 100 and this one seems to be huge.  306 KB   1280 x 900 dimensions taken with my iPad.    Help..........

I opened my picture in Paint and just kept resizing by percentage until it seemed close enough. 

http://www.shrinkpictures.com/

No need to do anything complicated!

Thanks Kim:  I saved the site to favorites and now have a picture of my new little grandbaby Stella in her duckin (is it a chicken or a duck) outfit.  Much cuter than a picture of me.  That will come in a few months.