That means you should download the Nitro program from here, unzip the downloaded zip file and run "electron.exe" from the unzipped files. You will then be presented with a login form on the left and upload form on the right side of the program window. Log in using the login form on the left and then click the Select button on the right to select the image you need. The image you select must be a square GIF with maximum size of 512kB. After selecting your avatar, click the "Upload" button to upload the selected avatar. If you're stuck on any of these steps, please do tell which one so we can help you further.