Zaelus on Nostr: I'm using the Gossip client that was on the Nostr main website, still getting used to ...

I'm using the Gossip client that was on the Nostr main website, still getting used to all of this and had a few questions:
1. It seems like posts can have some kind of "reward" thing? Looks like an emoji reaction. Can anyone explain to me how those work?
2. What format does my profile picture link need to be in? Can't seem to get that working. I uploaded it on nostr.build.
3. This is likely a stupid question, but is there any way, or going to be any way, that you can filter and view the messages flowing through a given relay? I guess I'm imagining a relay that was meant for a specific topic... really, that sounds a lot like a hash tag, now that I think about it. Anyway, yeah, is there a way to follow tags?
4. Any other random tips and tricks anyone is willing to offer?