Extras allow full members to post up to 100 additional images each calendar year. They renew on 1st January for paying members.
It allows you to post extra images alongside your post of the day, for when one image is just not enough.
If extras are available to you simply click on the plus sign inside the box below the words 'extra photos'. The page shows you how many extras you have available to use before 31st December.
If you find you have used up all of your allocated extras in a particular year you can delete older extra images from the same year, to free up space.