I didn't dig too much deeper, but since this client is HTML based I'm almost sure you could find the source graphic files and edit them; with the permission of SquirrelMail, of course.
Customizing webmail is done with the following command:
(in terminal as root)
Configuration-map: /usr/share/squirrelmail - command = ./configure. The LOGIN foto must be pun in the map /usr/share/squirrelmail/images with the name web_mail_login.jpg.
Both group and user for WEBMAIL = nobody. Use the command chown -R nobody:nobody /usr/share/squirrelmail to fix this if permission errors occur.