It looks like redactor is adding a rel attribute to images as they are added to content via the RTE. Removing the rel attribute and then switching views adds it back in again.
How can I remove the rel attribute as it's affecting validation: https://validator.w3.org/nu/?doc=http%3A%2F%2Fgrowthwise.com.au%2Fnews%2F2015%2Fgetting-organised-for-tax-sucks-is-there-an-easier-way