Gallery posts aren’t supported natively. You’d need to create a gallery externally and link it from Lemmy (possibly as an image post so there is at least a preview of the first image in the gallery).
It’s a frequently requested feature. There are some existential performance and mod-tool priorities that I think will get most of the attention of the core devs, but maybe someone from the community will step up to code this.
Microblogging is the name for things like Twitter. You "blog" only small texts like the famous 160 characters of Twitter. A tweet or toot would be called micropost or just "status update". On microblogs you usually don't have threads but replies to a post. These replies and the initial post effectively form a thread but all the replies are also separate posts by the user.
Microblogging is tweeting, or tooting, or similar. Very short (micro) one-way messaging (blogging). The term “blog” is an abbreviation of “web log”, which is simply a series of someone’s thoughts posted on the internet for whomever happens to pay attention. Readers are mostly passive.
A thread is a collection of comments from people actively replying to one another, usually organized in a tree structure. It’s more like a conversation.