An SMS step sends a text message. It is the most effective and the most intrusive thing an automation can do, so it deserves more thought than an email step.
Length and segments
The editor counts characters and segments. Messages over 160 characters are sent as multiple segments — which costs more per message and is worth knowing before you write four sentences.
Aim for one segment. A text that says one thing and gives one link is better than a paragraph anyway.
Images
Images can be attached, which sends the message as MMS. Because not every recipient can receive one, you choose what happens when they cannot: send the text without images, or skip the recipient.
Sending without images is almost always right — the message still gets through. Skipping is only sensible where the image is the message.