PhotoRoom never deletes a photo from your phone's photo library (the "Photos" app). However, when you import an image in PhotoRoom, it will create a project. To delete a project, open the "Your Content" tab, and click on the three dots on the top right of the project.

To delete multiple designs at once please try the following:
1) Go to "your content"
2) Next to "your designs" click "select"
3) select all the designs you want to delete
4) Click "delete"