Images can be submitted to game entries, publisher pages and designer pages. If the image is accepted by GeekMod the user is rewarded with 1 GeekGold. Read more about Image submissions.
Images can also be found in the Images Module which is shown by default on the Front Page. Each game may have a representative image.
Note that every BGG user also has a personal gallery (accessible from one's profile or via My Geek / Gallery) where the user can directly upload personal pictures not intended for moderation and appearance in a game entry.
All images hosted at BGG can be easily inserted into forum comments and other places using their image ID (visible in the URL of the image). See Forum formatting for details.