You'd need to download the image. The easiest way is to select a token and hit Z to pull it up, then right click and download it. Then you need to load it into an image editor (I'm using Affinity Photo, but Gimp and Photoshop are pretty much the same) Next, adjust the Canvas size to be larger. The token I did last night was 400x398 pixels, I made a 500x500 canvas--it's easiest if it's square. Next, I grabbed the drag tool (looks like an arrow, or possibly a 4-pointed arrow) and grabbed the layer to move it around. I like to center the nose of the token in the canvas. Then save out the file as a PNG and drag it into the VTT.