A term from classical rhetoric: the activity of describing pictures with words. The Oxford Classical Dictionary defines it as "an extended and detailed literary description of any object, real or imaginary".
Ekphrasis also has a narrower meaning, related to the vivid verbal or textual description of a work of art - for example, all we have of Greek canvas painting is ekphrastic descriptions.
One famous example of ekphrasis would be "Kubla Khan" by Samuel Taylor Coleridge.