Here are the instructions for replacing existing SalesLogix splash screen logo with a custom logo.
The default logo is 600 wide x 400 high
1) Rename base_splash_screen.png file in C:\Inetpub\wwwroot\SlxClient\images\icons
2) Paste new "base_splash_screen.png" into C:\Inetpub\wwwroot\SlxClient\images\icons
3) Refresh browser
The steps are similar to change the top left navigation bar icon
SalesLogix_logo_sm.png which is 147 wide x 25 high
Note - You may need to clear the cache from your browser before you can see the changes.