no, it isn't. and imageshack is free, unlimited, and easy to use.
everyone needs to quit bitching
I use the same URL for every web forum I go to, and to update my avatar I just FTP and overwrite it.
I have to be a member of a web site and go out of my way to update my avatar on here.
I have been an admin on several different message boards and have never heard of this kind of rule. I cannot understand a single "security" reason for not being able to link an image onto this forum unless the boards themselves are seriously poorly designed and can somehow allow cross site scripting just from linking an image.
And Chicken is still allowing images to be linked from anywhere on the web in posts, so I don't see how there's less of a risk now if there ever even was one.