One of the really cool things about Tabletop Club is that you can create, modify, and add your own (or someone else's) assets in the game using your favourite image editing / 3D modelling / text editing software, just by putting them in this folder!
All you need to do is drag the asset pack into this folder! Then, when you next play Tabletop Club, the game should automatically detect the new assets and import them so you can use them in-game!
For information and tutorials as to how you can make your own assets for Tabletop Club, have a look at the documentation.