I have edited your post to put the image inline. Please use the photo icon to insert images within the post. Seeing an image inline with the text makes it easier for anyone answering or viewing the post.
I am not familiar with the Mac OS but that shouldn't matter. to move images you drag within RoboHelp.
Here an image I want to move is in assets/images.

In Windows I would simply click on it and drag it to the required folder.

You can multi-select as well.
Does that help?
________________________________________________________
My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.
Use the menu (bottom right) to mark the Best Answer or Highlight particularly useful replies. Found the answer elsewhere? Share it here.