hoanganh17 - viewing an image in a browser is not the same as requesting it from a different web page. Many servers allow you to view images directly in a browser, but not request it from a different page (e.g. to avoid hotlinking or trying to steal content).
Use this tool to check what permission the server gives:
The first URL you try to load is this one:
The server responds with:
This means "you are allowed to use this image". This is because photobucket are OK with you using their images on other sites.
Try your other image URL:
The server does not respond with any "Access-Control-Allow-Origin" header. This means "you are not allowed to use this image", and the image fails to load. This is the default setting for most servers, or indicates the author of the site does not want you to use their images on other sites.
So this is still not a bug, you need to make sure you have permission to access the images you want.
Note in Chrome's console it returns an error message whenever you try to load an image you don't have permission for - I don't know why Firebug doesn't though.