In the 'Your Profile' section of 'Settings' there is a button toward the bottom of the page that states 'Deactivate Your Account'.
This will effectively close your account and remove all of your images and comments.
If you have a change of heart then you can sign back into the website using the original details within 30 days, and all your account will reactivate along with all your images.
If you would like to leave the site for a period of time but don't want to delete your images, you could make your account 'protected'. This will prevent your account from being viewed by users, unless you give them specific authorisation by allowing them as 'Followers' of your journal.
You can read more about Protected journals here.