Replacing the default ClearOS logo on the login pages and proxy error pages is very simple.
That image is being pulled from:
/var/webconfig/htdocs/templates/standard-5.1/images/logo2.png
First backup the file:
cd /var/webconfig/htdocs/templates/standard-5.1/images/ cp logo2.png logo2.bak
Now copy your image to that directory, and as long as it's named logo2.png it will load. The original image is 109x128. You can get away with a lot more width than that, but you need to keep the height the same in order for the login box to display correctly. There's plenty of ways to do the actual copy, since I'm already logged into the ClearOS server, I'll just scp the file from my linux client.
[root@clearos images]# scp user@system:/example/directory/logo2.png ./
That's it! Now go to your ClearOS login page and you should that custom logo.