I'll update this post as any changes to ways to post content in the forums is made.

Tagging Users

You can tag any user in the board automatically (linking to their board profile page) by simply typing "@" followed by their displayed username.
Eg. @Jerry

Images

~~Though uploading images to the board is not available~~ (see bottom of this section), you can post images in your posts by referencing the image location's remote URL using the Add an image button in the editor:

If you don't already have the image hosted somewhere online, you can host it on any one of several free image hosting sites, such as PostImages.org or ImgBB.com.

Just make sure you're using the actual image URL when you use the code to insert the image into your post. Depending on which service you use, you might have to investigate where the direct URL is.

For example, if using PostImages.org, you would want to use the Direct link URL provided, as shown in this image:

One key sign that tells you if it's the direct link is if the URL ends with the image file's extension, which is png in this case.

Note: Uploading images directly in the post is now allowed using the following button in the post editor:

Math Type/Rendering

MathRen has been restored on the site! Thanks, FreeFlarum!

\KaTeX

While some of the following sites can be used to assist in generating the syntax needed for formulas, etc., you can also refer to \KaTeX 's own supported functions page to get an idea of how to generate formulas that you need.