A picture of placidity? (5)

I believe the answer is:

still

I'm a little stuck... Click here to teach me more about this clue!

'a picture' is the definition.
(I know that still is a type of picture)

I cannot understand how the rest of the clue works.

(Other definitions for still that I've seen before include "Silence" , "Tranquil" , "source of spirits" , "Immobile" , "Not going anywhere" .)

I've seen this clue in the Evening Standard.

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