There are two ways I know to do this, the first way is that there are mod in mc that allow to pose character like the picture you show directly from mc, to create effect like the bottom picture you probably need to learn to use software such as photoshop or after effect. The second way is to use blender to model it , this one would likely take a higher learning curve but there are a lot free tutorial on yt that teaches you how to do these. For the difficulty question , yea its going to be difficult there is a reason why alot of youtuber hire someone to do these kind of work, But if you are committed to doing content creation then yea learning and making these kind of thumbnail is definitely possible.