Relative paths and private filesystem

When using relative paths and Drupals private filesystem Insert generates relative URLs which point to "sites/default/files" instead of "system/files". This results in broken images.

I think the problem is in the theming funktion "template_preprocess_imagecache_insert_image" in "includes/imagecache.inc". Attached you find a small patch which solved the problem for me. Maybe someone can review the patch if there are any side effects.

Regards, Tim