It should be self explanatory. Go to "your account," chose the avatar button and you should be given a dialog screen for uploading. Most people screw up by letting any one dimension be more than 150 pixels or the total file size greater than the limit which specified. The site does not re-size pictures.
Re-size it on your computer and save the smaller image file and upload that. Windows has an image viewer that I believe can re-size, otherwise you would have to download any number of simple image editors that are absolutely free.