Adding illustrations can really improve your UI and can help you add context to your project. There are many libraries that you can use and most of them are free.
Illustrations in design
Illustrations are an amazing way to add more personality to your design. You can use these in place of images, you can even visualize data using illustrations. These can also be used to relay an abstract idea. To see how illustrations are incorporated in a design, you can head over to Dribbble , Behance or Ello.