well, you could link the images to a non https address, but browsers will show that warning
"warning, some parts of this page are not secure. display them?"
the only thing that can cache anything from an ssl connection is the browser itself. intermediate proxies cannot, because its encrypted.
so all you can really do is make sure your server outputs the proper expires headers when it delivers the images. browsers should just cache the images anyway, but maybe this is different through ssl. im not even sure if a browser will cache them at all if its ssl, so you might wanna check on that.
still, i would try to optimize your images as much as possible to reduce file size, or consider using less or smaller images