If you have no preview images in ImageCache under Drupal 6.x than is's possible that you do not have clean URL Enabled. ImageChache doesn't work with Clean URL Disabled.
Check: Admin --> Site configuration --> Clean URLs.
If "Disabled" switch to "Enabled"
It's possible that you can't switch this is when the radio buttons are disabled. In most cases is that when mod-rewrite in apache is not activated.
Check:
ls -la /etc/apache2/mods-enabled/There must be a line with a symbolic link to rewrite.load -> ../mods-available/rewrite.load If not? Create it:
cd /etc/apache2/mods-enabled/
ln -s ../mods-available/rewrite.load
apache2ctl gracefulDone!
