Learn Arabic (Modern Standard): تلك صور جميلة.

This is the core rule of non-human plural agreement in Arabic! Whenever you have a plural noun that does not refer to humans (like صور for pictures), Arabic treats it grammatically as a single female. Because of this rule, any adjectives describing it or pointing words used with it must take their feminine singular forms.

Doesn't تلك usually mean "that" instead of "those"?

Yes! By itself, تلك means "that" when referring to singular feminine nouns. But because we are talking about a non-human plural, we are forced by the grammar rule to use this feminine singular pointing word. We translate it as "those" in English because we are pointing at multiple things, even though Arabic grammar treats them as a single female unit.

I don't see the word for "are" in this sentence. How do we know it's a complete sentence?

Arabic does not use a present tense verb for "is" or "are". When you put the pointing word تلك next to an undefined noun like صور (pictures), it automatically creates a complete sentence. The "are" is simply built into the structure of the language and understood from context.

Could this sentence also translate to "Those are beautiful pictures"?

Yes, in fact, "Those are beautiful pictures" is the most direct translation! Because صور (pictures) does not have the Arabic word for "the" (ال) attached to it, the invisible "are" lands right after تلك. If the sentence had said تلك الصور جميلة (adding "the"), it would strictly map to "Those pictures are beautiful." Both translations express the exact same idea.