Must check image styles for existence and get max of possible.

This is indeed not supported, the easy fix is to define image_100 the same as either image_auto or image_200. The problem with automating this is that it will depend on the particular use case.

Changing to feature request so I can think about it for a couple of days.

@emit.it3 i know it's a lot right now to do by hand, I'm going to add a form that will duplicate styles so you can select an existing image style and it will duplicate it for all defined suffixes, but i need to find the time ...

In the latest dev version you can now go to admin/config/media/resp_img/create_style and add responsive styles in a more user friendly manner

The thing is that this module relies on all the styles (one for each suffix) being present. The actual functionality boils down to this function:

/**
 * Replace suffix if needed / possible
 */
function resp_img_replace_suffix($path) {
  $bestfit_suffix = resp_img_get_best_suffix();
  $default_suffix = variable_get('resp_img_default_suffix', '');

  if (isset($bestfit_suffix) && !empty($bestfit_suffix) && $bestfit_suffix != $default_suffix) {
    $path = str_replace($default_suffix, $bestfit_suffix, $path);
  }
  return $path;
}

which basically does a string replace of the default suffix with the suffix that is best for the current window with.

Because it's just a string replace, we can't check if that image style actually exists...
Marking this as fixed (because of the easier way you can now add image styles).