Learn Russian: Обложка рисуется молодым художником. - The cover is being drawn by a young artist.

In Russian passive sentences, the person doing the action (the agent) is expressed using the Instrumental case with no preposition. Here, молодым художником is in the Instrumental case to show who is drawing the cover.

Why does the verb рисуется have the -ся ending?

The -ся particle makes this verb passive. Рисует means '(he or she) draws', but adding -ся to an imperfective verb makes it passive: рисуется means 'is being drawn'. The cover (обложка) becomes the grammatical subject receiving the action.

Why does the adjective молодым end in -ым here?

Because the noun it describes (художником) is masculine and in the Instrumental case. For most hard-stem masculine adjectives like молодой, the Instrumental ending is -ым.

Could I just say this in the active voice, like 'A young artist is drawing the cover'?

Yes, you can absolutely say Молодой художник рисует обложку. In fact, everyday spoken Russian strongly prefers the active voice! This passive version using -ся and the Instrumental case sounds a bit more formal or written.