No, you do not need to upload releases. When agreeing to the Submitter Agreement you accept to provide such releases upon question. In relation to the keywording of all clips, you must provide information about releases, and these answers are the basis for what kind of use the clips will be offered for (editorial use or commercial use). So if the clip needs a release and you confirm/answer that you have secured it, you are legally binded to provide that release if asked to by Clipcanvas, so make sure you store them in a secure place.