You can mask your images and videos either on the 'Advanced Editing' screen or the 'Edit Scene' modal.
Here is the process you need to follow:
Step 1: On the left panel, go to Enhancers >> Masks.
Step 2: Drag and drop a mask that you like onto the screen.
Step 3: Drag and drop your media onto the highlighted area of the mask.
Step 4: Adjust the mask by clicking on 'Edit Mask' on the right panel.
Step 5: To delete the element, either hit 'Delete' on your keyboard or use the trash button on the right panel.
Here is a short video to help you through the process: