What happens when you try visiting the links directly. Like:
https://bitcointalk.org/Themes/custom1/images/on.gifInterestingly this issue occurs with Opera 12.15 on multiple operating systems, as well as IE 8 on windows XP. It does not occur when using Firefox on both Windows and Linux however.
Based on this, my first guess is that it has something to do with SSL. Firefox handles SSL natively -- all other browsers rely on Windows' SSL handling.