I suppose, "shrinkToFit" class and the whole HTML code in the Inspect Element section of Firefox, are generated internally by Firefox and they do not have something common to miniBB nor to any other web software. Proof: open your forums following the direct URL, not the iframe'd version, like http://www.boernerc.org/minibb/index.php, then locate the topic you mention and you will see images are present with the only direct links and not kind of any HTML tag. Then if you click some of the links, it will point to its direct URL with the .jpg extension, and if you Inspect Element of this image, you will see basically the same code. Naturally, JPG could not generate any kind of HTML code, so this is purely Firefox' interpretation. The same could be seen also on our forums, for example, if you open this image (the direct JPG) and Inspect Element of it, you will see Firefox even adds two classes - "decoded shrinkToFit" which are in no way associated with the website which generates the HTML content, because there is HTML content, which obviously generated by FF itself for binary sources.
I'm not sure - why would you need to worry about all of this stuff? It doesn't affect the working of your website. You should more look in the HTML source, not in the source of Inspecting tools.