If you see an image on the main site, it's probably being hosted by the site and therefore can be used easily. Here's Lance:

I got this by going to the Dragon page and right clicking the sprite I wanted to use to open it in a new tab. Then I just used the URL as I would a Pokemon's and added it here! These follow the same naming conventions as the Pokemon sprites. Lance's, for instance, is https://img.pokemondb.net/sprites/trainers/gold-silver/lance.png
.