The imagemanager and tinymce converts URL tro relative URLs by default. But they can be configured to use absolute URLs. Check the relative_urls init option of TinyMCE and MCImageManager. If you set that option to false, all URLs will be absolute, you could also add remove_script_host to remove the http://host/ part.

Do you have:

<script language="javascript">
mcImageManager.init({
   relative_urls : false
});
</script>

<script language="javascript">
tinyMCE.init({
   .. rest of init ..
   relative_urls : false   
});
</script>

The first one tells the Image manager to return absolute URLs back to the form values within the TinyMCE dialog. The secound one tells TinyMCE to convert any relative URLs to absolute URLs.