Need Help in Image Upload mode in display specified thumbnails

[TypeEE]

I am trying to provide a static thumbnail for importing RSS feeds. For example, I have a "rssThumb.jpg" sitting in the images/ directory. I have also updated the rss import php to put rssThumb.jpg into linkfield1 during importing. However, it would not display the thumbnail when the link is displayed. When I rename the thumbnail to img_141.jpg (141 is the number of the link id) then the thumbnail is displayed.

I started to dig into the image_update_main.php

        // Get Image filename
        if ($main_smarty->get_template_vars(module_imageupload_filename_field) == "")
        {
                $imageFile = module_imageupload_fullsize_prefix.$storyID.".jpg";
        } else {
                $imageFile = $main_smarty->get_template_vars(module_imageupload_filename_field);
        }

where module_imageupload_filename_field is defined as linkfield1 in image_upload_settings.php.

Can someone help me to figure out why it is not displaying the thumbnail?