Learn Finnish: Huomio, älkää kuvatko täällä. - Attention, do not photograph here.

First, we use älkää, which is the plural or formal "don't". The main verb kuvata (to photograph) is a type 4 verb. To find its negative imperative form, we take the infinitive, drop the final -a to get the stem kuvat-, and add the ending -ko (or -kö based on vowel harmony).

If the affirmative command is kuvatkaa (do photograph), why does the negative end in -ko?

In the plural imperative, affirmative commands use the ending -kaa or -kää. But when making it negative, the main verb changes to the connegative form, which always ends in just -ko or -kö. The negativity is already carried by the auxiliary verb älkää.

Is älkää only used when speaking to multiple people?

Usually, yes, it addresses a group ("don't you guys"). However, it is also used as a formal command when speaking politely to a single adult. Because this sentence is a public announcement ("Attention!"), the plural/formal älkää is used to address anyone who might be reading or listening.

Why use täällä instead of tässä for "here"?