The Attached Images module is in the Post modules category, which appears when you're editing a Singular-type themer layout in Themer.
The Attached Images module displays all of the images from your media library that are attached to a single Post.
Under the hood
The Attached Images module is a "shortcut" module. It's actually a Gallery module with a field connection set to Post attached images in the Photos field. When you open this module for editing, you can change the settings for layout, image size, and spacing, just like a regular Gallery module.
The field connection requires that the Source field be set to Media Library because that's where WordPress attachments come from.