The Gallery holds every image, video, and design in your Storrito account, including anything you upload and anything you build in the editor. Folders are how you keep that list manageable once it grows past a screenful.
The Gallery has two tabs at the top, Gallery and Archive. Everything below happens on the Gallery tab.
The folder panel runs down the left side of the Gallery, headed Gallery/. Click the new folder button in the top right of that panel and Storrito asks you to Enter a folder name. Type it and click OK. Campaign names, months, and client names all work well, since those are the groupings people usually search by later.

Click a folder to open it. The heading becomes a path, so Gallery/New Releases/ tells you where you are, and folders can hold folders of their own.
There are two ways, and they do the same thing.
By dragging: drag the item onto the folder you want it in.
By selecting: tick the checkbox in the top left of each item you want to move. A bar appears along the bottom showing how many you have picked, with Move, Archive, and Delete. Click Move, choose the destination folder from the list, then click Move here.

The selecting route is the quicker one when you are filing a batch, since the counter lets you gather everything first and move it in one go.
Folders are most useful as a template store. Duplicate a design you expect to reuse, keep the copy in its own folder, and you can open it, change the parts that differ, and schedule it without rebuilding the layout each time.
This works particularly well for recurring formats, like a weekly poll Story or a fixed announcement frame.
Right click the folder name and select Delete.
Deleting a folder only removes the assets from the Gallery. It doesn't affect your scheduled creatives. To stop a post from publishing you cancel the post itself, which is covered in How To Cancel Or Delete Stories.
The two solve different problems:
If the Gallery is cluttered with old campaigns you no longer schedule from, archive them rather than filing them. See How To Archive Old Stories.
You do not need to leave the Gallery to schedule. Click any item to open its options and choose Social schedule under Post / Schedule to social platforms, which opens the scheduling page with that media already loaded. See How Scheduling Works In Storrito.