The source code for Photoshop v1.0.1 is now available at the Computer History Museum website
From the Adafruit Industries blog:
Photoshop v1.0.1 Source Code Released
With the permission of Adobe Systems Inc., the Computer History Museum is pleased to make available, for non-commercial use, the source code to the 1990 version 1.0.1 of Photoshop. All the code is here with the exception of the MacApp applications library that was licensed from Apple. There are 179 files in the zipped folder, comprising about 128,000 lines of mostly uncommented but well-structured code.
Photoshop v1.0.1 Source Code Released