As is known, the earliest example of guns, your matchlocks and percussion cap rifles were not really the most accurate things in the world, so aiming was seen as more "point in the general direction and hope for the best" more than a tool to increase accuracy. This would carry over into WW1, while the rifles were indeed much more accurate, to the point of aiming allowing soldiers to hit targets from great distances with great, well, accuracy, the military high commands of that era were sluggish to realize just how industrialization had changed warfare - it was not until the interwar period that there was serious experimentation on how to perfect the new mode of war, and to discard the old Napoleonic era of tactics in favor of what we generally recognize as modern warfare.
This stance in pic 1 being used is an old school sport shooting stance. It's actually a very accurate stance, as it puts the fulcrum of the rifle in line with the shooters center or mass. But, it's an impractical position for warfare, as it requires to be both standing upright as well as fully still. Though, at the time nations started to field snipers, the majority of them were trained by or just were hunters and sport shooters - many of the USSR's snipers were just country boys who enjoyed hunting turning their skills into tools to destroy fascism.
Another adaptation of the era was pistols - though they had existed prior, their mass utilization by infantry was entirely new, and there was very little standardization of shooting stances for accuracy. What ended up getting adopted by most nations was largely their "officer's stances", which was a position that was once used for holding aloft officer's swords, before pistols became the status symbol of the office. Pic 2 is the American army, pic 3 the Germans, and pic 4 the English. The only concern of this stance is getting the shot on target, it was popular with dueling and target shooting. If you've seen videos of people shooting like this, you'll know there's very little recoil control - making it impractical for warfare due to difficulty of follow up shots. You can see an example of these kinds of hold-over firing positions in this video - https://www.youtube.com/watch?v=jP7J-JNSUu4
A lot of these old stances have no modern equivalents in warfare, but others are the ancestors to our modern forms.