I believe that one's imagination is feed by drawing and painting from life. In that way you retain a memory of images to draw from of the variety of shapes in nature and attitudes of people and a ton more stuff to consider. Imagination is different from memory, a modern day person would have an impossible time remembering what it was like to experience the events at Bunker Hill.
I am posting some images from my sketchbook that were drawn from imagination as well as some quotes about imagination by artist and other well known persons.
“I am enough of an artist to draw freely upon my imagination. Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.” Albert Einstein
When being praised for a painting he had just finished, the artist, James Fitzgerald said, 'You should have seen it in my mind.' Guy Corriero
"A picture is first of all a product of the imagination of the artist; it must never be a copy". Edgar Degas
"A person without imagination is like a teabag without hot water." Alan Fletcher
"There is no better way of exercising the imagination than the study of law. No poet ever interpreted nature as freely as a lawyer interprets truth." Jean Giraudoux
"Do not quench your inspiration and your imagination; do not become the slave of your model."
Vincent van Gogh
"Imagination abandoned by reason produces impossible monsters; united with it, she is the mother of the arts and the origin of source of their wonders." Francisco de Goya
"An artist who has no imagination is a mechanic." Robert Henri
"No amount of skillful invention can replace the essential element of imagination." Edward Hopper
"It is not enough to know your craft – you have to have feeling. Science is all very well, but for us imagination is worth far more." Edouard Manet
"I saw the angel in the marble and carved until I set him free." Michelangelo
-on painting July Sunset...
"This is not an actual scene, I made it all up... I drew it from my imagination rightside up, then painted it upside down, then corrected it rightside up to make it believable. It's all about the mood." C. W. Mundy
"Imagination will often carry us to worlds that never were. But without it, we go nowhere." Carl Sagan