Yes, ILLUSTRATION is a Scrabble word!

Letter values in Scrabble US: I 1, L 1, L 1, U 1, S 1, T 1, R 1, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

ILLUSTRATION is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

artwork that helps make something clear or attractive
showing by example
an item of information that is typical of a class or group
a visual representation (a picture or diagram) that is used make some subject more pleasing or easier to understand