Some sites don't allow you to link to their images. Could that be the issue?

is exactly right.

this is because web standards dictate that an https web page cannot embed materials that are served over http. most browsers won't even allow it. so since subeta is now served over https, http images will not work. if you look at the page source code, you'll notice that even though you used an http link for that photobucket image, we've embedded it in the page with the https link. which is why it works. tinypic apparently doesn't serve images over https, and there's really nothing we can do about that. :/

locking as there is nothing we can do about this.