I've found two seemingly incompatible definitions of "system image" (surprisingly both by same author)
"system image": the information conveyed by the physical product itself. (Originally published in Norman & Draper's User Centered System Design (1986), and reused frequently thereafter: The Design of Everyday Things (1988, 2003) and Emotional Design (2004).
How do we form an appropriate conceptual model for the devices we interact with? We cannot talk to the designer, so we rely upon whatever information is available to us: what the device looks like, what we know from using similar things in the past, what was told to us in the sales literature, by salespeople and advertisements, by articles we may have read, by the product website and instruction manuals. I call the combined information available to us the system image.
Page 31 of The Design of Everyday Things (revised and expanded edition) by Don Norman