Have you got Norton Firewall installed? This is the most common reason thumbnails don't appear. Basically the Norton/Symantec firewall sees the thumbnail images as web-banners and blocks them, even though they are legitimate web page items. This is a "feature" of the Norton firewall software, however there is a way around this.

This Symantec support article details a resolution:

Manually configuring Ad Blocking in Norton Internet Security, Norton AntiSpam, or Norton Personal Firewall

You will need to follow this, to permit anything with the text "fotopic.net".

Specifically:

Permit strings You can create permit strings that allow Web sites to display images that match the string. This allows you to override the blocking effect of any string in the (Defaults) block list for individual sites. Permit rules take precedence over Block rules on any site.

To test if this has fixed the problem:

1. Close all loaded web browser windows.
2. Open a fresh web browser window, enter your gallery URL then click go.
3. This page should load correctly. If this fails to load, try refreshing the page while holding down the "shift" key to load an completely fresh copy of the page.