On our web site you can upload up to 20 images at a time by simply dragging and dropping them to the pink "Add photos here" field on the Upload Photos tab.
Alternatively, you can do the same after opening a specific mission if you want to contribute to a certain theme.
For our mobile apps you upload to specific missions. Tap on a mission and select upload.
Remember to tag and describe your photo for users to find it! Thats important if you want to earn money.