Once you add many pictures and text to your collage, it might become difficult to take mouse actions such as dragging pictures without modifying text or vice versa. You can lock pictures to take actions on text without having to worry about modifying the positions of pictures by mistake. Similarly, lock text to take actions on pictures without having to worry about modifying the positions of text by mistake.
Note that once you lock pictures you wouldn't be able to take mouse actions like dragging, scaling, and rotating. You can still shuffle your collage or change collage properties using the Settings tab in the panel on the left of the application.