Every project is built with a collection of images associated to it. To associate images to your project, add them under the Photos tab. The photo collection pane is displayed at the bottom of the screen. This pane can be used to track the used and unused images in your project.
By default, images that have been used in your project will not be displayed in the photo collection pane. This allows you to more easily focus on the unused images. To show all the photos in the collection, click on the Menu icon in the photo collection pane, and deselect the "hide images used in project" option.
In the collection pane, images that have been used once in your project will be marked with a green tab in the corner. Images used two or more times will display an orange tab.
Note that the used/unused status applies only the current project. It cannot track image use in other projects.