How can I credit an Original Image on my thesis?
In the paper:


  Plotnik, J. M., Brubaker, D. L., Dale, R., Tiller, L. N., Mumby, H. S., &amp; Clayton, N. S. (2019). Elephants have a nose for quantity. Proceedings of the National Academy of Sciences, 201818284.


Figure 1 is an illustration of the experimental approach. The caption for Figure 1 ends with the statement:


  Illustrations by Nuttayapond Doungcharoen (artist).


There is no citation in a reference list because there is no prior publication, and I would not recommend citing as a private communication or anything like that as suggested in another answer (which gives credit but makes clear that you are paraphrasing some element of a conversation), it's a picture that the reader can see, you just want to attribute that picture to the artist.