Mac Screenshot Markup/Annotation with Text that Sticks to Arrows
If you open your images in Apple's Preview, you can use the toolbar to add arrows, and text:


Then you can command-click each of the elements to move them respective of each other's position.


All of these annotations were done with Apple's Preview. You can do the same thing for PDFs as well.

And of course, taking screenshots are incredible easy in MacOS. cmd-shift-4 with  a space bar afterwards lets you take pics of windows.