Quetzalcoatlus is not often shown on the ground like they are in this illustration. There have been skeletons posed in a quadrupedal stance on the ground, but typically they are not illustrated as animals that can even be on the ground let alone animals that live on the ground at any point in time ever. However, to go in line with the theory that these large pterosaurs ate small animals whole or in small torn up bites, they would have to find the ground at some point as their feet were not made for grasping and carrying prey items. Witton, here, has gone with that theory of prey items, the small baby Alamosaurus making a rather tasty snack it would appear to our Quetzalcoatlus foragers. Other theories, other than the piscivorous diet have also been put forward. One theory holds that Quetzalcoatlus may have been a strainer feeder, like modern ducks except instead of settling on the water and dabbling Quetzalcoatlus would have swooped low like a frigate bird and scooped then siphoned water to strain out plankton and other small creatures for consumption.
The reason that I have included the Martin and Shiraishi illustrations in flight today are because they are extremely similar and notably different at the exact same time in ways that are very important anatomically. The ways they are similar are, naturally, their large clear wingspan and the gliding nature of their flight. Granted the pterosaurs had the ability to flap their wings but a gentle glide is typically mentioned as the preferred mode of flight for most pterosaurs given the lesser chest muscles in comparison to modern birds which pterosaurs possessed. The major difference in these two animals as illustrated is their crests. There is preserved bony crest associated with some of the finds attributed to Quetzalcoatlus, however, the full crest of very few pterosaurs is actually known given that the soft tissue above, around, or protruding from the boney appendage could look like anything.
Shiraishi opted for a long backward protruding crest which is highly typical of pterosaurs, and quite stylish as well. Martin's illustration went with a buzzcut-like crest. The actual boney protuberance looks much closer to the illustration done by Witton, however, according to all of the skeletons that have been shown on display that I have found. Regardless, both Martin and Shiraishi's illustrations are equally well done.