If you're using Twine for the First Time:
What if I want to add pictures to my Twine?
To make a long story short, this is the number one reason to use Twine 1.4.2. You can use externally-hosted images in Twine 2, or use a base64 encoder to add images to your game, but because Twine 2 is .html-based, it can't store images as binaries. Twine 1.4.2 complies a .html website from code ("twee") so you can add images directly to Twine 1.4.2 and see them in your project.
Why not just use Twine 1.4.2, then?
Twine 2 is a clear improvement in terms of ease of use, and as long as you're not learning bad habits, it's easier and faster to get started with. Also, a Twine game is fundamentally textual: images can add something, but you may want to stop and ask whether your story is improved by illustration. Poorly chosen illustrations in a Twine can mar it, similar to the effect of opening up a novel only to find that someone had scribbled on the pages.
What if I want RPG mechanics, fancy text, or other bells and whistles?