Version 1.95 adds the ability to define a custom logo on the login screen:
To define a custom logo for your Webmin/Usermin installation edit the theme-stressfree/config file and set the logo parameter to the valid URL of the logo you wish to use.
e.g. logo=http://www16.plala.or.jp/bend_tail/images/ubuntu-logo-transparent128.png
Note: No image rescaling will be applied to the logo image specified. For best results the logo should be approximately 300x125 pixels in size (or less). For graphical consistency it is recommended that transparent GIF or PNG logo images be used.
The revised theme can be downloaded from here.