All real artist can paint / draw anything they want (or anything that anyone wants) right from their head.
Contrary to popular belief not all artists can create life-like images straight from memory. Actually, the majority of artists all work from some kind of reference - whether it be a live model, a still life, or photograph. It is VERY difficult to paint / draw realistically without some kind of reference.
This is me trying to draw a realistic figure, any kind of car, most animals, or fabric - without a reference.
This use to really bother me, until I actually thought about it. For me to draw or paint anything and it look like the real thing, I would have to know exactly what it looks like. Think of a shoe. You know what a shoe looks like sure. But do you know what the pattern on the laces look like? Do you know what the fabric of the shoe looks like when it's folded just a little bit? How about the stitching? If you actually think about it, there are lots of things we "know" in our heads, but we really don't have that many details about them saved up. HOWEVER.. give me a shoe to look at, and I will copy it. Whatever my eyes can see, my hand can draw. Give me a photograph.. and shit's gettin real.
Appreciate the artists you know for their own unique style and process, & if you are an artist - do the same to yourself! It doesn't matter how you get there, only where you are when you're done.