Why would you exit? On a Lisp Machine you can make the image and continue working. It also has incremental images, etc.
If I save and load an image, the new definitions are still there.
What you refer to has very little to do with imags, Smalltalk keeps a database of code in files under system control. That's also how Interlisp worked.
On a Symbolics Lisp Machine I would configure the machine, load my software and create an image. After starting that image I would on login load any changes from the code bases. If this would be too long, I would save an incremental image.
The Lisp Machine had many more image related features...