Learn Finnish: Älkää kuvatko salamalla. - Do not photograph with a flash.

Because this is a plural or formal command. For a singular, informal 'Don't photograph', you would say Älä kuvaa. But for plural or formal commands, you use Älkää and add -ko or -kö to the consonant stem of the verb, giving us Älkää kuvatko.

How do we find the stem kuvat- from the verb kuvata (to photograph)?

The plural negative imperative uses the verb's consonant stem. Kuvata is a type 4 verb. To find its consonant stem, you drop the final -a, which leaves kuvat-. Then you just add the -ko ending to make kuvatko.

Why is 'with a flash' translated using the -lla ending in salamalla?

The -lla ending (the adessive case) is used here to indicate a tool or instrument. While you probably first learned that -lla means 'on' (like on a table), it is also the standard way to say you do something 'with' or 'by means of' an object. So salamalla means 'with a flash'.

Does salama also mean lightning?

Yes! Salama originally means a bolt of lightning. The word was later adapted to also mean a camera flash because of the sudden burst of light. Context usually makes it obvious which one is meant.