max-width for all images in the editor

Hello,
I can’t find the solution although i’ve been searching and testing the solutions on the forums. That’s why i open a new thread.

I use Tinymce advanced plugin and mailpress on the blog of a client.
I would like him to be able to insert full size images in the post he’s writing without having to struggle with sizes.

He would simply insert the images which would have a max-width. The theme i use already display images with a certain max-width.
I simply would like to have the same thing during the writing of the post.

Seems odd but even if i edit the content.css nothing happens…

Does anyone knows how to do that?

thanks!