The best way to get this is:
First:
U have to desing your own template, you can use the Dreamweaver Extension, its very easy to use and you can do whatever you want with your joomla.
You can download it and learn how to use it here:
www.mambosolutions.com/dw_tutorial/
Second and most important:
You have to use the template you have done "TWICE", You just have to copy your template folder and rename it:
Example:
templates/before_login
templates/after_login
Now that you have two templates...you just have to asing the "templates/before_login" to show only in the front page...you can get that by using the template manager...
Now go back to your Dreamweaver and modify the templates to your needs...
Example:
templates/before_login: create a cell with a background color or a background image, and then insert the left module into it...
templates/after_login: Do the same with the left module... But use a different position , color, background image etc
Remember to asing mod_login to show always in left module
We just need a little of imagination...thats why i love joomla !!!!
Hope u Understand...
Have a great day guys!!!