SBCL cannot save an image if there are multiple threads running, and Slime by default uses a separate thread for communicating with a listener. Unless you use an alternative communication mode saving an image from that state would be difficult. You could try arranging the core to save itself after killing swank threads, but I have no idea how to do that and trying things would probably just shut down the image.
Not that it matters if you cannot create one, but using a preexisting image from Emacs is much simpler. You can either add a "--core" argument to slime-lisp-implementation variable, or start a swank server (by loading swank, slime server component, and doing swank:create-server) and connect to it from emacs with slime-connect.