Create custom levels and share them with the Community. Or just browse, download, and rate awesome maps that others have created. Maps with a high enough rating even have a chance of becoming official! Click here to learn more.
When you're publishing your map from the editor, theres a field to choose the image. It must be a PNG and no lower than 2x2 pixels. You cannot change this image from Steam Workshop itself, it must be changed when publishing your map from within the Editor!