Because with Photobucket you only get a certain amount of bandwidth a month. When you've exceeded the allotted bandwidth for the free account, you get that photo. Use imgur.com, imageshack.com, or tinypic. They're free and don't give you that bull shite.
When mine did that, it was because I had used all of my bandwidth for the month. Either increase it, or use another hosting site. I've been using imgur lately. Have had no problems & seems to be quicker.