Two instances of Glowstone can be seen in this picture.
In Zombiecraft, a way of creating light without a torch is using glowstone. Glowstone is included in minecraft as a block that provides a light level of 9, but in Zombiecraft, the texture of cobblestone is used on glowstone as well. This is done to add an "ambient lighting" feature to the game by providing light without other blocks that obscure the player's sight and taking up space. This is a tool used in most of the Zombiecraft maps to help the player see much more clearly when shooting zombies.