The preview depicts an image that blends into the background colour of the sidebar. To achieve that I had to extend the original gif. The dimensions I found worked best with this theme are 220px by 300px, but you can choose what ever you like.
Manatopia did a great little tutorial on a how to extend an image into a background colour, and to make her tutorial work for a gif all you have to do is make sure that the gradient (or brush strokes, if you use those) are visible on all the frames.
An option to make the sidebar background repeat is there in case you opt out of the gif/image extended and choose to use a pattern.