We get constantly asked this question. Yes, we are glad that you are using the platform to create scientific images for your book.
You can use them under our Creative Commons license of use. The only thing we ask, as a retribution, is to mention the platform and include the link to the website mindthegraph.com in the figure legend.
Also, as you are a free user, the illustrations will have a watermark (see examples below).
One can always subscribe to get rid of the watermarks and get access to customized illustrations, which will certainly increase the quality of your work.