I appreciate all the responses on my query about The Official Website of
Mark Twain, or CMG.  I wrote to them using their web site contact but
heard nothing back.  I suppose using an image of twain in a 32x32 screen
icon is below their radar, or I did not appear serious enough about it.
To be sure of "my rights" to use public domain material in a commercial
venture I should probably write the Mark Twain Foundation's legal
department.  They have a snail mail address posted on the Mark Twain
Project's web site.  I still need to figure out how to get a USB Flash
drive to autostart in Windows 7.