Would the meaning of the context changed, if I use the adverb "quickly" in this context?

What does "snatch A from B" mean?

To snatch something means to grab or seize something quickly. When you "snatch A from B," this means that you quickly took something away from another person.

What's "scrape by" mean?

To "scrape by" means to live with barely any money- or enough money to get what you need and nothing more. To "scrape by" can also mean to only just succeed at something, or to almost fail. Ex: We scrape by with what we make at the market every week. Ex: Johnny scraped by on his driver's test. I'm surprised he actually passed.

What's "pay attention to" mean?

"Pay attention to" means to concentrate on, take notice of, or consider something. Ex: Pay attention to the people crossing the road when you drive. Ex: If you pay attention to the last scene, you'll see her walking through the door.

What does "hit on [someone]" mean? In what circumstances could I use it?

To "hit on someone" means to flirt with someone, or try to strike up a conversation in a flirty, romantic way. It's similar to the expression to "pick up someone". Ex: Someone tried to hit on me, but I told him I had a boyfriend. Ex: I have never hit on someone before because I'm too shy.

Can it be "when taking the shot" instead here?

Good question! You could say it, but it would imply something slightly different. Here, "to take" means the possible moment of taking a picture, whereas "taking" would mean within the process and action of taking a picture. So, if they said "when taking the shot," it would mean that while they were taking the shot, they weren't sure about something. "When to take a shot" means the right moment or time to execute the action. Ex: When editing photos, I like to make sure it's the right size first. Ex: I never know when to edit photos during the week. Ex: I hit my head when I was walking downstairs. Ex: I'm not sure where to walk so I can avoid the puddles.