Unfortunately the Editor does not provide advanced control over the HTML for the inserted images out-of-the-box. There is an existing feature request which would provide you the desired functionality. I suggest to cast your vote for it and follow the item's status.
I am sorry I cannot be more helpful here.
Feel free to ask if you have any further questions.
Progress is here for your business, like always. Read more about the measures we are taking to ensure business continuity and help fight the COVID-19 pandemic. Our thoughts here at Progress are with those affected by the outbreak.