This question is just a small curiosity.  I worked around the problem
with symbolic links.

I started a new image in a new directory and got a complaint about
being unable to find the .sources.  After some digging I found the
global variable SourceFiles, a StandardSourceFileArray.  I reset its
first element to 
MultiByteFileStream readOnlyFileNamed: '/usr/share/squeak/SqueakV3.sources'.

Though this worked temporarily, every time I saved the image I got the
same message about sources not being found, and subsequently
discovered a new instance of StandardSourceFileArray had been created
(with nils where the files should have been).

I looked through the code and even tried putting some halts in the
initialize methods for StandardSourceFileArray.  The latter hung up
the image.

Can anyone explain what's going on?

Thanks.

P.S. I also checked the archives.  My executable is not in the same
directory as my .sources.