A widely shared image appears to show a giant figure walking about the clouds. However the most likely explanation is that it is two plumes of condensation from a power station rising above a low cloud layer
It was originally posted on joe.ie
A similar mysterious plume was observed in January 2014, knowledge of the flight path, and the time of the photo allowed the exact power station to be tracked down.
It's likely that a similar finding can be made in this case if the photo time is known.
If you edit out one the of plumes from the "robot" figure, it becomes clearer:
A possible candidate is the Doel power station in Belgium, which is roughly on the Vienna-Gatwick route, and has two cooling towers. And has been observed to produce similar "legged" clouds before. Here's one from ground level.
However there are several similar power stations, and only knowing which flight he was on, and what time the photo was taken, will solve the puzzle.
Another power station along the route is the Großkrotzenburg coal powered power plant in Germany (Kraftwerk Staudinger). This seems like the most probably one to me.
Source: michi.p on Flickr
This is a more detailed image than the "giant", and shows just how low the cloud layer can be, just a couple of hundred feet above the ground. It's also right on the direct path from Vienna to Gatwick