If I navigate directly to /gun_register/norfolk_island/photos_norfolk_island/gun_norfolk_Island_18pdr.jpg, it comes up. But on the page (once one has clicked on the thumbnail) it's a broken image icon. I've seen this once before. In that case it was I believe a problem with permissions on the image file and/or its folder.
That's set server side, either in a .htaccess file or the server configuration.
There is a typo (red in the highlighted section - scroll the code block to see it):
But removing that extra quote mark didn't seem to help.
.jpg"" rel="thumbnail"><img src="photos_norfolk_island/gun_norfolk_Island_18pdr.jpg" style="width: 250px; height: 180px" /></a>