Hello;
let's say you want the unregistered visitors only to view this, instead of the thumbnail:
This file is called "denied.png", and attached below. Copy it to
templates/your_own_templates/images/ folder, or create your own one.
Change your template file "
thumbnail_bit.html" to this:
<!-- you wish detail page in a small javascript open window, use {thumbnail_openwindow} -->
{if user_loggedin}<div align="center">{thumbnail}<br />{endif user_loggedin}
{if user_loggedout}<div align="center"><img src="{template_url}/images/denied.png" width="150" height="150" alt="" /><br />{endif user_loggedout}
<b>{image_name}</b> {if image_is_new}<sup class="new">{lang_new}</sup>{endif image_is_new} ({user_name_link})
<br />
<a href="{cat_url}">{cat_name}</a><br />
{if allow_comments}{lang_comments} {image_comments}{endif allow_comments}<br />
{lightbox_button}
The tags {if user_loggedin} and {if user_loggedout} will help you to display the image thumbnail for members, and "denied.png" to others.
Please do the same changes to
templates/your_own_template/random_image.html and maybe
random_cat_image.html if you want to get non registered members unable to see random thumbnails. Don't forget to use the closing tags {endif user_loggedin} and {endif user_loggedout}.
Don't ask if it is possible to have a particular own thumbnail instead of the original one. It would be too much work for a poor result, I think.
Hoping it could help, regards.