Oh you went straight to hardcore nostr.
1. It seems you are talking about zaps. ⚡️ this is typically a lightning address to receive Bitcoin.
2. Almost all formats work. I have seen .gif .jpg both should work fine.
3. I’m sure those exist, but not very common. Hopefully someone can add more to this. #asknostr and yes you can follow tags, not sure if that’s doable on gossip, but most clients offer this.
4. Start with a more mature simpler client. Gossip is amazing still needs some time to evolve.