To subscribe to pictures you go to the workshop. You should see it right here where discussions is selected under the game's name, a bit to the right there is workshop.
To share something there are several buttons in the game at the top row. One of the buttons says Pic. It will open a folder at the game's location named Pic. This is where you copy your image. It should then show up in the game (it will refresh the pictures on it's own, scroll down to find it). Once it's there you can play it to test it. To publish, there is another button with an upwards arrow. This will upload the selected image and give one achievement if it is successful.