Help!

Ok blog world!

I'm getting ready to open a Teachers Pay Teachers store (you have NO idea how excited I am! :D). I've noticed that other bloggers who have a TPT store post these cute previews of the lessons. It has all the documents in one image so bloggers/buyers know what is in it. Either I'm really dumb or I just haven't learned the trick yet. Can anyone help me out and explain how to do it? :) Thanks in advance...

Sarah