Going over your monthly bandwidth will lead to overage charges, which is charged at $0.15 per GB. To prevent these charges, it's best to host your images / videos on your server.
It's not possible to go over your monthly image / video quota. Once you've hit the limit, you won't be able to create new content until your quota resets on the 1st of the following month. Alternatively, you can upgrade your account to a higher tier.