Smalltalk programs that run under gnu Smalltalk are linked with the system classes in gnu Smalltalk class library. Therefore, they must respect the terms of the Lesser General Public License1.
The interpretation of this license for architectures different from that of the C language is often difficult; the accepted one for Smalltalk is as follows. The image file can be considered as an object file, falling under Subsection 6a of the license, as long as it allows a user to load an image, upgrade the library or otherwise apply modifications to it, and save a modified image: this is most conveniently obtained by allowing the user to use the read-eval-print loop that is embedded in the gnu Smalltalk virtual machine.
In other words, provided that you leave access to the loop in a documented way, or that you provide a way to file in arbitrary files in an image and save the result to a new image, you are obeying Subsection 6a of the Lesser General Public License, which is reported here:
a) Accompany the work with the complete corresponding machine-readable source code for the Library including whatever changes were used in the work (which must be distributed under Sections 1 and 2 above); and, if the work is an executable linked with the Library, with the complete machine-readable "work that uses the Library", as object code and/or source code, so that the user can modify the Library and then relink to produce a modified executable containing the modified Library. (It is understood that the user who changes the contents of definitions files in the Library will not necessarily be able to recompile the application to use the modified definitions.)
In the future, alternative mechanisms similar to shared libraries may be provided, so that it is possible to comply with the gnu LGPL in other ways.
 Of course, they may be more constrained by usage of GPL class libraries.