Everything that is not a question goes in through the same / or + menu. Here are the three that come up most: images, tables and code.
Images
Type /image, or pick Image from the + menu. You then have two options in the empty block:
Upload image, or drag a file straight onto the block. JPG, PNG and WebP are accepted.
Paste a URL to an image that already lives on the web.
Images work inside a question prompt as well as on their own, which is what you want for "label the diagram below" or "which of these graphs shows...". Put the image in the prompt itself and it stays attached to the question wherever the question ends up.
Worth knowing: the auto-marker reads images that appear above a question on the same page, so a question that refers to a diagram is marked with that diagram in view. See How long-answer questions are graded with page context.
Tables
Type /tab, or pick Table from the + menu. Click into any cell and a toolbar appears with everything you need:
Insert row above, Insert row below, Delete row
Insert column before, Insert column after, Delete column
Merge / split cells, for a header that spans two columns
Toggle header row, which also tells screen readers that the top row is headings rather than data
Delete table
Drag a column border to resize it. Column widths are kept for students, so a table you have tidied stays tidy when they open the lesson.
Tables are read as page context too, so "using the data in the table above, calculate the mean" is a question the auto-marker can actually mark.
Code
Type /code, or pick Code Block from the + menu. Syntax highlighting is automatic and covers the languages that come up in teaching: Python, JavaScript, TypeScript, Java, C, C++, C#, Ruby, PHP, SQL, Bash, CSS, XML, JSON and Markdown.
A code block preserves your indentation exactly, which matters if you are teaching Python. Paste code in rather than retyping it and the whitespace comes across intact.
The rest of the menu
Also in there: video and YouTube embeds, document attachments, links to other pages in the lesson, external links, interactive visuals, dividers, headings, lists, blockquotes and the five callout styles. See Inserting content with the slash and plus menus.