While creating your project, you can import as many photos as you'd like, but photos need to be less than 16 MB in size. In order to keep your file size down while creating your project, you can remove any unused photos in one action.
Instead of removing all unused images, you can instead remove photos one by one. To do this, be sure the Media Tray is showing your photos and then click the Manage Photos button to open the Photo Manager. You will see a red "x" for photos that are not used in the project and a green check for photos that have been used in the project. You can click the red "x" to delete that photo from the project.
If the photo is used in the project, you will need to delete it from the page first. Click on the green check for the editor to jump to that page, close the photo manager and the click the photo on the page. Once that photo is selected, click the delete key on your keyboard or the red "Remove Photo" button in the upper left. You can then go back to the photo manager to delete the photo from the project.