I can't see the image button mentioned in the knowledgebase (see image above).Instead, I tried to paste an image into a Note with Ctrl+V. This seemed to work, but when pressing Save the image is removed.
Unfortunately, for the moment images are not supported for notes in the Leads Inbox. This is however definitely something in our roadmap, even if with no current ETA.
Apart from Leads Inbox, you should see the option to add an image for all other items. If you don't, then kindly reach out to our support team so we can investigate further.
Wish you a lovely week!