So as long as I store my pirate software in my browsers cache folder I'm fine?
No. Reason: pirated is pirated, no matter where you store it. You can't legalize what is not legally available on the net by storing it inyour browser cache.
With media like images, that are publicized on the web, things are different. First: they have been made public for the consumption by the public. Second: you can tell a browser not to cache your content. Third: even if the browser caches the image, the source is always clearly visible. Fourth (and most important): your browser is not redistributing the images over the web.
Discourse, on the other hand, makes a local copy and then distributes that copy as if it had the licence for distribution, giving no indictaion whatsoever of original ownership of the work in question.
This has nothing to do with copyright laws in far-away countries like Germany - this is a huge problem even in Discourse's homemarket of the US. The image posted by @tufty won't get Alex into prison (he can always cite prior publication), and the US government might not sue for damages re its intellectual property, but an enterprising lawyer might see ample opportunity to exploit Discourse's behaviour:
- Find a photographer who could need some extra revenue
- Find someone to post a link to a specific artwork by that photographer
- Wait for Discourse to make local copy of that artwork
- Document that this site is serving said artwork as its own
- Sue for damages
I don't know for how many Discourse installations the feature of making a local copy is turned on (it wasn't for BoingBoing last time I checked), but the more sites are careless enough to use this "feature", the greater the chance of a lawsuit coming up.