You may change the rule in bb_codes.php, removing .(gif|jpg|jpeg|png)
pattern for [imgs] and [img] parts, but then the codes insertion will become absolutely insecure. All normal images must end with their extensions; that rule was introduced because of security terms. Basically, I think, re-copying images like this is a bit out of website rules. In a timely term, they also could change this address and make it unavailable, it looks like a temporary cache or something like this.
There's another, more convenient way of storing such images: copy the image itself on the disk, then re-upload it on forum, using either File Bank
either File Upload
add-ons. Storing images locally is better in all terms.