From Mod Wiki
An Advanced Map
1. Introduction and goals of the tutorial
2. Necessary map declarations
3. Caulk hull
4. Creating a simple terrain
5. Adding an atmosphere
6. Adding player spawns
7. Adding base structures
8. Adding base vehicles
9. Creating a constructible Bridge
10. Adding an MCP escort objective
11. Adding a Shield Generator hack objective
12. Creating a destructible objective
13. Adding Objective spawns and vehicles
14. Info Objectives
15. Deployzones
16. Playzones
17. Masks
18. Barebones Map script
19. Creating a command map
20. Levelshot

The levelshot is the screenshot of the map shown on the loading screen. In ETQW there is also a smaller "thumbnail" version displayed in the server browser.

You can easily make a levelshot by just taking a screenshot of your level from a nice angle in the game. Take this from a 4:3 aspect ratio and screen resolution (eg. r_mode 4 and r_aspectratio 0) or else it will look stretched in the loading screen.

Once you have your screenshot, you will need to scale it down to a reasonable texture size - typically a 256x256 pixel image. This will make the image look a little squashed in your image editing application, but don't worry, the game will display it at the correct aspect ratio.

Make a 128x64 pixel version for the levelshot thumbnail.

The recommended paths and naming conventions for levelshots are described in the necessary map declarations section of this tutorial.