I don't see anything wrong with the picture.
Now if they were Americans I would go on about how clearly unsafe they are, how they have fingers on the trigger, how the pistol in the right hand of the pistol guy is pointing at shotgun guy, etc
How they are playing into pop culture and in unrealistic foolish poses with a serious piece of equipment. Someone cannot even use two pump action shotguns at a time, and we all know duel wielding pistols is for video games and movies.
In America guns are normal, and people should know how to safely operate common normal items that are a Right. Mishandling should not be tolerated.
However they are Australian, so none of that really applies because regular people don't have access to guns there. Those that do go through an extensive process and learn how to use them for sporting purposes only which will teach them how to handle the firearm (and they will be locked in their state approved safe unloaded when not target shooting at a formal location.)
Expecting an Australian to know how to properly handle a firearm is like expecting everyone here to know how to properly operate a random pick piece of industrial machinery. It is something most people will never be around, and so have no reason to ever need to know how to safely operate. Sure it would be nice if every person knew how to safely use every piece of machinery, but unfortunately that is not possible.
As for offensive? I see it as two tourists taking part in something foreign. They are not even shooting them, just posing with something common often lamented in other nations. Americans have this false image as gun crazy cowboys in some parts of the world, so posing with some guns in America is just kinda playing into that for viewers back home.
It is a 'was there and did that' image.
In fact the irony of it is they are probably making fun of guns, innocently of course. Because people in most such nations are raised to view guns as quite bad, and the notion that regular people can easily obtain firearms is seen as foolishness. A concept they probably partially hold themselves which makes holding them up feel a little naughty and so is part of the attraction to do so while in America. Yet it is still innocent touristy fun, even as people who are representatives. Getting some pictures holding guns in America is an iconic foreign culture picture.