You can download
www.irfanview.com for free; it's very good. As Z-man said, crop it as square as you can. In the Image menu, select "resize/resample" and resize it to 50x50 pixels, or limit the larger dimension to 50 pixels if it's not square. Before you resize it, increase the DPI to say 200. Once you've resized it, open the Image menu again and select "sharpen." In the File menu, select "Save As" and leave the "Save quality" at 100 to start. The avatar needs to be 20K maximum in size.