Provide setting to choose image shape and allow images to be responsive

edit: sorry, premature response.

This is how I first did it. I wanted to add the .img-responsive class Bootstrap uses to the image field. I installed the Image Class module (and it's dependencies), and added the class to the field.
But I ended up using a preprocess function in my subtheme to add that class to all image fields: Read more here.

That seems to work fine, thanks!

There seems to be a problem in image.vars.php at line 17 ... I get the following php error :
[] operator not supported for strings

This breaks my sites pretty badly (WSOD).

That is because some module is passing the class attribute as a string instead of an array (they should always be an array). You should backtrace which module (obviously one that also preprocess images) is causing this and file an issue there.
https://www.google.com/search?q=drupal+class+operator+not+supported+for+...

Thanks Mark, it turned out to be languageicons. I'll submit a patch there...

This needs to remain open.

I'm just moving this back to 7.x. If this needs re-evaluation in 8.x, create a new issue.