Our game assets are free because of donations from the community, consider one if you can!
To use custom textures create a new folder in /Textures/, pick any name to describe the type of textures you want to import (i.e. "Stone" or "Wood"). Place the custom textures in the newly created folder. You will have to restart Asset Forge in order to see the added textures.
The recommended size for custom textures is a 512 × 512 PNG file.