1. I'm afraid I don't know anything much about the img-responsive class other than I believe its part of bootstrap and allows the images to resize with the screen width resizing. I found it in the html after adding an image in Atto, but it seems to disappear if the image is given a specific size in the image upload box.
2. Tables are intended for just that - tables. They are not intended for layout - http://www.w3schools.com/html/html_layout.asp
However, previously with TinyMCE and currently with Atto (as far as I can see at the moment) without going into the html and creating everything in code, tables are just about the only option for many teachers creating layouts for their content. What we probably need is some sort of layout plugin/template that makes adding the bootstrap grid, or other layout option (flexbox? - when it achieves sufficient traction in browser adoption), straightforward for 'non-techie' users - something that allows proper responsiveness and is as simple to use as inserting a table. I'm really hoping that the fact bootstrap is used in core now might make that possible - but the Atto plugins are currently something I have not looked at.
3. @Alan "Resizing images outside of Moodle prior to upload is too complicated for most instructors. What appears to be the resizing options currently within Atto are also too complicated - requiring an instructor to exit Moodle and re calculation of the proportional ratio."
a. Resizing images outside Moodle is an essential and if it's too complicated for your tutors to upload an image to a site such as webresizr.com to resize then you may need to think about taking the content creation responsibilities off them, because if they start uploading full size digital photos without resizing them they are literally going to kill your site!
b. Atto handles the proportional calculations itself - change the width and the height adjusts automatically if you simply tick the 'Keep ratio' option. But even that only changes the display size - if the original image is a 4000x3000px 5Mb+ image, then that is what is uploaded before the css kicks in to resize it, every image, every time.