There need not be a single image. You can save and load as many images as you like. In fact, you can have thousands or hundreds of thousands of images if you prefer. You may think I'm being uncharitable to the author by construing his words as I did, but it's entirely appropriate given the tendency of those ignorant of Smalltalk (like him) to speak and act as if there is some monolithic, fragile image that everyone on a project shares and each must strive to avoid breaking.
The Smalltalk image may seem unfamiliar or even scary at first, but it's really little different from a standard Ruby or Python process, except that it is serializable and contains all of the code (both compiled and in source form), data, and execution state as objects that you can manipulate.
Further, every Smalltalk (that I know of) supports saving and loading source code to text files and often some form of package management and source control as well. Many Smalltalk also integrate with file-based tools quite well. For example, you can use git with Pharo, and many Pharo developers use Github exclusively for their development.
People citing the image as a reason for not using Smalltalk clearly don't know Smalltalk. They're also likely hypocrites, because at some point or another they've no doubt asked themselves why, after so many years, are programmers still programming by the mere editing of dumb text files? Well, in part because of your hostility to any system that isn't the mere editing of dumb text files.