Collectible Card Game Headquarters

Hello,
how can I use cropped images with renders in latest Forge, instead of full art?
I've downloaded a bunch of 511x397 cropped art via Gatherer, but don't know in which folders to place them nor how to name the files.
I've tried the "cards" folder, a "crops" folder, both the ".full" and ".crop" extension, and no extension at all... but it doesn't work - the renders in Forge show no art at all.
Someone could help, please?
Tx

Anyone can help?

If I well remember, you have to use ".artcrop" as extension, not ".crop"

Thank you!