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...
In any case, at least a few Schemes have images, so it's a moot point in that debate. It's still interesting, though.
Oh, something like LispWorks has extensive support for using images, too. It has a very extensive application/library delivery mechanism based on images.
LispWorks can also save 'sessions', even automatically as periodically timed events. Even without quitting Lisp. Sessions are basically IDE states (files, listener history, lisp image, windows, ...).
Anyways, Lisp images don't work like that: a lisp image is a static preloaded environment. If you redefine some functions, exit lisp, and reopen the interpreter, those redefinitions won't be there.