1. While playing the adventure, you can click "C" to take a picture. (You can do it in the editor, but then the hud is covered in junk). The screen will flash and you will hear a camera noise if it successfully took a picture.
2. In the editor, click on your adventure name, and it should pop up where you can change description, tags etc. Below this there should be 4 pictures. All 4 of them are set to the planet view from space, the default picture.
3. Beside each picture, there should be arrows to move through the pictures you took. You can have up to 4 pictures to showcase your adventure, and you can take as many as you want. These 4 pictures are showcased when you view an adventure in the sporepedia. (Picture #1, or the top left corner picture, will be the thumbnail)
If you want to set custom pictures (your own picture files on your pc) as your adventure thumbnail, take a look at this thread: http://spore.boards.net/thread/89/change-gas-adventure-pictures
