Images of originality
Any diagrams, figures, or images in your thesis must be treated in the same way as your text: they must be your own, or they must be appropriately cited. However, unlike text, when an image is copyrighted (and it is safe to assume that all are unless it is explicitly stated that they can be used with a citation only), then you must obtain permission to use someone else's image.
The over-use of figures downloaded from the internet detracts from the quality and it is preferable to see the candidate's own images, which then would be clearer and in some instances more appropriately targeted to the relevant information.