True. The “broken page” icon in my image is actually rendered by the browser itself, not the website. On my desktop computer, for example, it just replaces the icon with the alternate text (“TouringPlans Discussion Forums”) without the icon. That is a browser decision. But the CAUSE of the issue is that the link to the image itself is broken. What is weird is that if you scroll up to the top of the page, the image returns properly. I think, then, the bug is in the code that modifies the heading bar. When you first go to a thread topic, the heading bar only shows the Touring Plans logo (which links to the categories page) because you can see on the screen the thread title.
But if you scroll down, the thread title disappears, causing the forum software to modify the heading bar to now show both the TouringPlans logo PLUS the thread title. But it is in this modification where the link to the image is broken, and so we end up only with the alternate text.