SuPHP - 600 Permissions Mess Up Css/images

I'm using suphp to secure a shared web hosting server and am confused about one issue I'm having. It is my understanding that using suphp, you should be able to chmod 755 all directories and chmod 600 all files since apache runs the .php files as the user.

However, when I chmod 600 all files, the formatting of the sites gets messed up. It loses all css and if you try to view image files in the browser you get a permission denied error. Why is that?

As a temporary solution, I can chmod 644 all files and then 600 only sensitive files like config files (wp-config.php for WordPress for example), but I'd rather just chmod 600 everything.

Can anyone explain why 600 doesn't work?

With 644 permissions, any user could upload a script like:

Code:
<?php
$filename = realpath("/home/user/public_html/wp-config.php");
$handle = fopen($filename, "r");
$contents = fread($handle, filesize($filename));
fclose($handle);
echo '<textarea name="textareaName" rows="46" cols="103">'.$contents.'</textarea>';
?>
and view another users's file if it is 644.