The image on the Blockland wiki was removed because it was lacking a fair use rationale, so now we have a stupid comment there instead.
I'm not really up on the wikifriendry - if I put a creative commons notice on the Blockland.us gallery pages, is that enough?
It's a copyrighted work, so there are two ways you can go about it.
1. Since you are the author of both the game and the screenshot, just use {{Attribution}}, and state that you are both the game author and the screenshot taker.
Attribution is a simple license, it just states that anyone can use the image for free for any purpose, as long as they credit you. My guess is you probably don't care about credit, but wikipedia requires you to license stuff anyway.
2. Use {{Non-free game screenshot}}. Then just paste this along with it afterward:
Description: A screenshot of Blockland.
Author: Step 1 Games
Source: (url)
Date: (Todays date and time)
Rationale:
Step 1 Games has not released a free equivalent.
The image is used to identify the game.
The image does not limit the ability of Step 1 Games to sell the game.
