Go to local game, right-click on the map to open the editor, select the desired terrain and generate a new map. Look for a double island one, so that you can screenshot one of the islands and it will fit in the canvas on it's entirety.
Don't forget to press the "preview" button so you can see the map with the objects and everything. On most terrains it's basically impossible that you're able to get all the objects on a single island, but just try to generate one that looks interesting and makes the terrain properties shine ;D
Once you get a nice map, go back and add a team with a single worm, and change the scheme settings for manual worm-placement (teleport) and disable mine and drum objects.
Start the game. Place your worm on the opposite island that you want to screenshot.
Press the Insert key until you get black background and 3 waves
Then, move your mouse and try to make the island you want look horizontally centered on the screen. Don't worry, it doesn't have to be perfect, you can adjust later.
Take a full screenshot of the game and open it with Photoshop or any other editor. In my case, this generated a document with a canvas size of 1920 * 1080.
Change the canvas height to 1010 pixels, with the anchor on the top (Image > Canvas Size)
After that, change the canvas size to 940 width * 840 height, with the anchor on the bottom
If needed, move the image horizontally to center the island
Manually erase objects from the opposite island in the screen, if any Use the magic wand with 0% tolerance and Anti-alias disabled to select all the black background and remove it. Don't forget to do it for closed gaps inside objects as well.
Done! Save as a .png and upload it to the site, edit the page and follow the format in the gallery section of the other pictures to add it :)