Referencing is necessary to ensure you do not plagiarise (copy) someone else's work. You cannot present the work of someone else's as your own. This applies to the writings, ideas, statistics, images and audio recordings of other people, whether it is published or unpublished. You can avoid plagiarism by citing your sources.
Try these excellent tutorials on how and why to reference:
Some 'free for education' or open source material for images. Use this list to ensure you are respecting Intellectual Property and Copyright. Note: always double-check the terms before use