Finally nailed down the "correct" image size for pets: 170 x 150 pixels, PNG format (no lossy compression), with about 10 pixels padding, on a white background. This will not only look good in the info boxes on the side, but more importantly, the Wikia system will pick it up as the first image on the page that is larger than 130x115. Therefore, they will be used for the article thumbnails, and (now comes the clever part) the aspect ratio is the same as 130x115, so they look "perfect" when scaled down and/or cropped!
Right now, when you go to (for example), there's a wild variety of random images, some have no thumbnail. Doesn't look good at all. But if you look for the German Shepherd or the Lemon Lime Gecko, you will see nice, perfectly cropped images! All the rest will look like this (eventually) :-)
The only thing to keep in mind is that the article thumbnails (and preview text) is refreshed daily, otherwise it comes from the cache. Therefore, when you make a change, it will show up the next day.