How do I assign custom art to a custom card?

Hi there... kind of a side question but I was wondering how do I assign art to a new card?
One of my hobbies is creating cards and a friend has a whole set, so I decided to teach myself how to code in Forge with custom cards (given that the "remaining" cards are WAY over my head).
So I was able to figure out how to add a new set and a new card (which you guys make easy to figure out) but for the life of me I cant figure out how to get the right art to come up for this creature.
Every reference I see to the art in the available files points to online links rather than the 3 letter folder with the card images. Obviosly peoples side projects is not a priority, but if anyone knows off the top of their head I would appreciate it.
One of my hobbies is creating cards and a friend has a whole set, so I decided to teach myself how to code in Forge with custom cards (given that the "remaining" cards are WAY over my head).
So I was able to figure out how to add a new set and a new card (which you guys make easy to figure out) but for the life of me I cant figure out how to get the right art to come up for this creature.
Every reference I see to the art in the available files points to online links rather than the 3 letter folder with the card images. Obviosly peoples side projects is not a priority, but if anyone knows off the top of their head I would appreciate it.