This is because direct links to photobucket images are not links to images but rather a link to an .html file disguised as a .png. Have you tried manually visiting a direct link? About one out of every four times it'll go to a page with just the image, the rest of the time it's to a photobucket page with ads and links to the direct link which is, ironically, what you were trying to visit.
And Mysidia trying to access these files is no different than a person trying to. Sometimes it'll go to a direct image, sometimes it won't. If it gets an image file, no error. If it gets the page with ads instead, it throws the error you're getting.
My recommendation? Don't use photobucket. Alternatively,
pay to be a premium member on photobucket so you're actually allowed to hotlink images - free members only get so much bandwidth, and once you're past that, all you'll ever get are errors. Basically, hotlinking takes up their bandwidth, and
that's a bad thing, so they replace your images with an error so you stop stealing their ad revenue which they get when people view their images ON photobucket itself.