Placing Tiles
Clicking "Edit Map" will open the editing menu with the tool "Place" selected. The editing menu allows you to alter the map beyond what is procedurally generated. These changes update in real-time to all people viewing the map.
You can select a Terrain or Foreground tile and then click a space on the map to place it there.
You can also click a space on the map where you've already placed that tile to remove it.
Foregrounds will get placed on top of terrain tiles. They tend to be transparent and a bit smaller than terrain.
Next to both the terrain and foreground lists you will see "+" buttons. Clicking this will allow you to add custom tiles.
By clicking the gray circular foreground tile, you can erase foregrounds, whether they were set during your session or not.
To make larger sets of edits to the map, check out Using The Line And Box Tools as well as Using The Magic Tool.