That is if you are using the embed image icon from the toolbar; if you type directly, you bypass the tag recasting that that tool imposes if you forget to untick the box.
In other words, context is everything. If you directly type your tags like me (I'm an old school html hacker, and so remembering tags isn't that hard), all you need to remember is to use a fully qualified URL/URI. I have found over the last two years of using vBulletin at various sites that it is sensitive to not being told what protocol to use in image/audio and file links (http/ftp/https/etc) - no protocol, no embedding. simple as that.