In the same manner your mind can reproduce an unlimited number of
pictures. Anything which you have once seen with the physical eye can
be reproduced again in the mind's eye.
Make a few tests of this fact, if it is not well known to you. For
example,--
See a pasture with a creek flowing through, willows hanging over the
water, the green grass on the banks, and the stock grazing there. See
several different kinds and sizes of animals, note their color, what
they are doing. Add to the detail of the picture.
To close the eyes and thus to eliminate the more distinct impressions
of the physical eye, will assist you in visualizing any picture.