I could tell you how to do it but it would probably kill you.
No really, the simplest solution is to ask a kid for help - almost anyone under 20 can probably do it for you in a jiff.
If you want to do it the long way:
1) If you don't have one already, search for "free photo editor" and select one to use (Photo editor online / free image editing direct in your browser - Pixlr.com
) is an example
2) import the picture into the program
3) look for "EDIT" in the top line menu and select "image size" or use the "help" option and lookup "resize"
4) follow the instructions to change the images' size
5) SAVE or EXPORT the image in .jpg format (suffix to the file name) AND GIVE THE FILE A NEW NAME (so that you can go back to the original if needs be)
6) now you should have a file that you can load into your profile