Yeah, basically what others said :).
As Vincent mentioned, I’ve been building my apps with two “databases”: Solid and browser storage (Focus uses localStorage and Media Kraken uses IndexedDB). But I think at this point everything is very experimental. I’m the only one using Soukai that I know of, and I needed to modify it for many use-cases. It’s also true that I’m not an expert in Solid or Semantic Data, I’m still learning. But it’s possible.
In my opinion the only real requirement for an app to be compatible with Solid is that it’s modeled with RDF. Like Mark mentions it’s not strictly required, but if you don’t do it your app won’t be interoperable with other apps. Which is, in my opinion, the point of Solid. I guess if your app only manages a bunch of images you don’t need a lot of modeling, but it’s not usually the case. For example you may have albums of photos and it’d be nice that they are modeled with RDF.
When it comes to querying the data… Yes, that’s a big problem at the moment. It’s the reason why for Media Kraken I cache all data in the browser. I started a discussion about this a while ago in case you want to take a look: State of the art for querying large containers.