Definitely you, or your system. I post images reqularly with no problem, switch between the RichText and HTML entry panels, and always preview before submitting.
I'm using FireFox 28 - odd issues like yours tend to be with stored pages (cache) and cookies, so If I were you I would log out, clear your cache, clear your cookies (all those containing Autodesk anyway). Close FF, re open it then try again. Also make sure you allow cookies and JavaScript (if you're using NoScript or similar) for any of the Autodesk sites (and a couple of lithium sites) - they do need it.
A "few years ago" the forums were entirely different and contained completely within the Area, now they are a sub-section of the main Autodesk forums which uses very different software.