This article describes how your work is automatically saved to your Hub. (Read more: What is Hub?)
Images are automatically saved to Hub upon upload
When you select an image to edit, before it is opened in the Editor, you'll be asked to name it and sort it into a folder in your Hub. Click Create. It is now saved in your Hub.
Changes are automatically saved as you work
The image you are working on in the Editor will periodically save as you edit. You will know that the latest changes/edits are saved when you see "Changes saved" in the upper right corner of the Editor.
If instead of "Changes saved" you see "Saving now..." Hub is working on saving your image, so wait a couple seconds.
Typically though, updating changes to Hub is nearly instantaneous.
Edit a Copy when you don't want to write over your past work
When you want to work on an image, but aren't sure you're ready to commit to changing the original, select "Edit a copy" near the top of the editor. This will copy the work in it's current state to a new file, and load that file in the Editor: preserving what you've already done as a separate image. You can do this at anytime while you are editing. You can also opt to Edit a Copy before you even get into the editor. For more on that see Opening Image You've Worked on Before.