When images are generated they are not automatically saved into your asset library. This is to enable you to maintain an asset library that only contains the assets that you want and not have it filled with discarded assets. This is especially important given the AI models draw upon your asset library when responding to prompts.
Unless generated images are manually saved/added to the Asset Library, they will remain in the relevant projects that you have created.
Saving a generated image to the Asset Library happens inside the Ad Editor.
It is not currently available from the Works page.
To save a work as an asset:
Open the ad (or any ad in the same workspace) in ads editor.
In the left panel, open Images and select the Work tab. Generations will appear in the grid.
Click the three-dot menu on the work's thumbnail.
Select Save to Assets.

