For Penguin mail 1.6 the logo file used is site/templates/templatename/images/logo2.png
For music city 1.6 the logo file used in the demo is site/templates/templatename/images/style1/logo.png
To make modifications to the styling etc. for both templates you need to edit site/templates/templatename/css/system/error.css file.
Edit: forgot this bit.
For your hosting accounts general error handling you could create a standalone 4xx/5xx .php file. Because not all the errors on your account would use the joomla error control system.
Here is some info on
creating a 404 script.
If you don't fancy that the you could write some simple html error pages or shtml if you want server side includes.
An easy way to replicate the joomla error page would be to use your browsers view source function, while viewing the error page. Then cut and paste the relevant part/s into an editor and include any additional styling in the doc head or in-line and save as an html/shtml file. In your Cpanel you can set-up the error page handling using those files.