Please be sure to post images using HTTPS and not HTTP.
Example of embed code using HTTPS (secure):
Code:
[ img ] https://i.imgur.com/OMrxLdK.jpg [ /img ]
Output is a green padlock which indicates page is secure and encrypted.
Example of code using HTTP (not secure):
Code:
[ img ] http://i.imgur.com/OMrxLdK.jpg [ /img ]
Output is an orange padlock which indicates page contains elements which are not secure.