I'm not sure the 'danke' explanation quite fits. Originally it ment 'dank' as a term for pot or weed, and thus, most pictures were marijuana related. This stretches as far back to 07. But sometime in around...'12-'14, people started posting pictures that were cool on a different level, and not nessissarilly pot related.
Cool colors, or a dark theme tend to also define 'dankness'. Meaning that in a dark room, you're not going to have blaring windows 10 stock background type images.
At this stage, it tends to mean a level of 'cool yet interesting', where you can stare at the picture, and keep finding something meaningful within the more you stare, but that it also make for good background.
Hope this wasn't to wordy for you.