Learn Spanish (Latin America): Las fotos son gratis.

Foto is actually short for fotografía, which is feminine. Even though it is shortened and ends in o, it keeps its feminine gender, so we use the feminine plural article las.

Since las fotos is plural and feminine, why doesn't the adjective gratis change to match it?

While most regular adjectives change their endings to match the noun's gender and number, gratis (free) is an exception. It is an invariable word, meaning it always stays exactly the same whether the noun is masculine, feminine, singular, or plural.

Can I use gratis to say I am free this weekend?

No, gratis only means free of charge (costing zero money). If you want to say you have free time or are talking about freedom, you use the word libre.

Why use son instead of están here?

We use son (from the verb ser) because the price, or the fact that something is free of charge, is treated as a basic characteristic or permanent attribute of the item.