Since we converted to https protocol in December, it has been advisable for members posting remotely-stored still photos (e.g., those hosted on sites like photobucket or flickr) to use the secure version of their resource URLs. That is, you should use https rather than http whenever https is available. Always includes the "s" in a resource links on your initial attempt. If the link then fails to bring up the resource it is pointing to, you can simply edit back to http. The reason that including the "s" is recommended is because the http addresses may not render your images for everyone due to protocol restrictions on certain browsers that are relevant to mixed http/https page content. When you are able to use https addresses, all will usually be well.
*Important Note* Please practice the reverse for your YouTube video short URLs when you wish them to embed within your post bodies. The reason for this is that whenever [video][/video] tags for a short-form YouTube link are used and the "s" is present, the video player cannot embed. To work around this issue, simply drop the "s" from https and the player should then appear.