Hide it how? By forgetting it, calling the police (twice), and having your otherwise boring art purchase make the international news?

Besides, wouldn't it be the seller that's liable for taxes? And if they're trying to hide from Israeli customs then (a) good luck with that, and (b) as far as I can tell, their plan was to... just travel to Israel with the painting? What's the plot here, exactly? I'd at least get someone to paint a few replicas and import them as a set.

Also tax evasion isn't moral.