Assuming you have a Windows computer (rather than a Mac), open your photo in Windows Photo Editor. You'll find it in the programs section in the Microsoft Office Tools folder.
Click on the Image dropdown menu and select Resize. Change the first number to something smaller (maybe someone else can tell you the right size for an avatar) - the second number will change automatically to maintain the picture's proportions.
Save the result under a different name and try uploading again.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan