In order to join a challenge, go to the "Open Challenges" section, select one of the challenges you would like to join and mouse over it.

Then press the 'Submit Photos' button to add photos from your device or profile.

Once you're done, press 'Submit' and you're in!

Please note that you are only allowed to submit up to 4 photos per challenge and make sure to read the challenge brief and rules before you select the images you want to participate.