Motivation
Currently, Save Image will always overwrite existing images. This is intended, but not always what users want. We have gotten numerous requests to add an option to not overwrite existing images.

Description
Add an option to Save Image that allows users to skip existing images. So if there already exists a file that Save Image would overwrite, Save Image should do nothing.

This option should not only not overwrite existing images, it should also automatically optimize the execution to not compute the image that won't be saved. Not computing the image is important, because this will enable use cases such as #1770.