whatever you do, please don't use dropbox with http links... use https. if you work somewhere that websites get blocked, there's a strong chance that file storage sites do, and the only way you might get around this is with an https link. imgur is definitely one of the more readily accessible and easy to site solutions for sure, although my preference is picasaweb since i'm a heavy google user.
if you want to embed the images without using the advanced editor, use the following code:
[img]http://i.imgur.com/blah.jpg[/img]