Learn Polish: Oni są sławnymi malarzami.

It is an agentive noun (a word describing a doer or a profession) derived from the verb malować (to paint). The suffix -arz is one of the most common ways to create profession nouns in Polish, much like the English suffix "-er". Other common examples include pisarz (writer) and lekarz (doctor).

If this sentence was about a group of female painters, how would it change?

It would become One są sławnymi malarkami. The pronoun changes to the non-masculine-personal One, and you use the feminine form of the profession, malarka. Notice that adding the feminine suffix -ka to malarz causes the rz to drop out.

Why is the phrase sławnymi malarzami used here instead of the nominative sławni malarze?

As a quick refresher: whenever you use the verb być (to be) to link a person to a noun describing their profession, identity, or relationship, that noun phrase must be in the Instrumental case. Here, we use the plural Instrumental endings: -ymi for the adjective (sławnymi) and -ami for the noun (malarzami).

Would it be correct to just say Są sławnymi malarzami?