I have exactly the same problem and Wizzard's reply doesn't help. Some hosts simply don't allow access to a file if it is linked directly from another site. Even typing in the URL for the zip into the browser returns a "page not found" error. Now with site software like 3dcommune, no problem. Attach the thumbnail to the URL of an html document on the host site that then links to the file itself. The downloader perusing 3dcommune clicks on the thumb, gets a remote page, and downloads from there. But the Renderosity software doesn't allow you to link a thumb to another page.