To add code blocks in a note, you can:
Go to Type > Code > (language) in the editor toolbar
If the language you want to use isn't in the list, type ``` to make an empty code block and Highlight.js will try to highlight it correctly.
Tip: Press Shift+Enter to escape from code blocks.
Tip: Long code blocks can be collapsed by hovering over it and clicking the "∧" button in the top right.
Images can be copy/pasted into your notes from your clipboard such as screenshots. Images from the web can also be pasted the same way if you copy it with "Copy Image" in your browser, or you can drag + drop.
If you want to paste in an image that you have as a file, open that file in a photo editor, select the whole image, and copy its data to your clipboard. Then you can paste it into Codex.
The "Insert > Image" button in the editor is only meant for web images with a URL right now. It'll be changed soon.
You can add tables by pressing Insert > Table on the editor toolbar. Once in a table, a special Table menu will appear on the toolbar for actions like adding/deleting rows and columns, etc.