The Wotja URL embedded in the Wotja button link includes a background image making it much bigger (450Kb).
This is the easiest approach if you want to include an image in your WJURL as it will play in Safari for Mac and iOS users can download it from the "Wotja" button weblink (seen only in mobile Safari).
It adds size to the webpage, though, so you might prefer to use an imageless WJURL in the page and allow a user to download the image-including WJURL via another page (next example).