。
Finders keepers.
Literal
Picked-up [past-た] person [subject-が] owner.
A 体言止め (noun-ending) proverb equivalent to English 'finders keepers.' The pre-nominal modifier 拾った ('having picked up') directly modifies 者 ('person') without a relative pronoun. 持ち主 ('owner,' literally 'holder-master') is the standard word for 'owner.' The whole sentence lacks any verb or copula — maximum ellipsis in service of a punchy saying.