Simple enough -
Enter your VZi User Name, select a GIF or JPG file from your computer less than 60K size and click Upload.
VZi User Name
File to upload
 
  A note...
When someone clicks the thumbnail the bigger image will be resized to 500 pixels on the widest side (either height or width). So if your image is smaller it may lose quality when resized. If it is bigger that doesn't matter.