[Plugin: WP Document Revisions] Failure to access image thumbnails

Hi everyone,
I’m building a frontend for the document revisions plugin, and it all went smooth so far but now I’ve run into a little problem. I would like to display public documents on my blog’s front page, even if the document is password protected. Every document has an icon displayed, the image is one fitting the document’s mime-type. This works very well. But now I would like to display the thumbnail image for every document that is an image (the thumbnails are created and I am even able to get the correct URL for the thumbnail displayd (the one with 150×150 in the file name), but when I try to display this with a <img> tag, only the full image is displayed. I’ve tried using the various functions associated with retrieval of attachments, but either the url doesn’t work or I just get the large image.
Any thoughts on this would be greatly appreciated!
Thanks,
Daniel