This is the 13584th most frequent Polish word.
pięknego
"Pięknego" means "beautiful" (masculine singular genitive or accusative form) in English.
The word 'pięknego' expresses the direct object of the sentence 'a beautiful cat' in the accusative case, masculine singular.
Mam pięknego kota.
I have a beautiful cat.
The word 'pięknego' here describes 'painting' in a negative construction, using the masculine singular genitive case.
Nie widzę pięknego obrazu.
I do not see a beautiful painting.
The word 'pięknego' modifies 'kształtu' indicating possession of a beautiful shape, masculine singular in the genitive case.