1) First make a nice custom world.

2) Once it's finished, ready to be published, save your work and exit the game.
(The Simplex World Creator will revert any change made to the preview picture if you save your project after having changed its preview picture)

3) a) For Windows7 users : Go to
C:\Program Files (x86)\Steam\userdata\22067611\253030\remote\user_levels
or
C:\Program Files\Steam\userdata\22067611\253030\remote\user_levels
if you are using a 32bit OS

b) For MacOS users : Go to
Macintosh HD>Users>Your_User_Name>Library>Application Support>Steam\userdata\22067611\253030\remote\user_levels

4) There you should find your XML file (the code of your world) and a PNG format image both named with the name of your world.

5) You just need to replace the PNG image with any other of your choice (screenshot, fan art, text...). Be sure to rename it exactly like the original one : nameofyourworld.png

6) Restart the game and go to the list of your local custom world.

7) Select your custom world of which you have changed the preview image. It should show it on the right.

8) Click the share button, follow the steps and there you go, it's done, your game is up with a nice preview picture :)

- The original thumbnails are images of 800x600 pixel. I recommend replacing them with images of the same format. Although I haven't tested with different formats :/

- If your custom world is already uploaded on the Steam Workshop, it's not a problem. Just follow the previous procedure and your Steam Workshop file wil update automatically, showing the new preview picture.

- Use image manipulation softwares like Gimp, Paint or Photoshop to crop your chosen image at the right size and to save it in PNG format.

- You can find your Race the Sun screenshots there :
a) For Windows 64bit OS users :
C:\Program Files (x86)\Steam\userdata\22067611\760\remote\253030\screenshots

b) For Windows 32bit OS users :
C:\Program Files\Steam\userdata\22067611\760\remote\253030\screenshots

c) For MacOSX users :
Macintosh HD>Users>Your_User_Name>Library>Application Support>Steam\userdata>22067611>760>remote>253030>screenshots




I hope this will be useful and will add up some more interesting preview pictures in the Race the Sun Steam Workshop :)

Rate up and/or comment if you found this guide useful or if you have any question.