Yes you can do them.
Have a look in the DreamHost wiki at http://wiki.dreamhost.com/index.php/404.
The page error.php is just a fancy page that could be replaced by a simple 400error.html file name. Just change the line in the file .htaccess which you would add to your web root folder (the folder with your domain.name).
There used to be a problem with IE whereby it would not display an error error.page unless it was at least 128 bytes long. In which case you would just add a comment line to pad the file out.
If you want to keep things neat you can create an error folder for your newly created error messages and images, but just remember to update the .htaccess file
ErrorDocument 404 errordocsfolder/error.php
Opinions are my own views and are not the views of DreamHost.
Any advice offered by me should be acted upon only at your own risk.