It depends on the image. My illustrations are mostly “freestyle”. If I draw something I need a reference to see shadows, a skull for example, I’ll set my model skull on my desk.
I get this question a lot, so just as a side note I’ll say I don’t see anything wrong with using reference for those who do. Nor is there anything wrong with making things up entirely from your mind. There is no rule saying either are acceptable or not. :)
So are you!!!