Put the inserted image inside a container so it can be easily positioned in text

I think it would be great if the inserted image would be placed inside span containers

For example:

A class to place the image in the horizontal middle of the page:

CSS code :

span.wysiwyg_imagefield_floating_none {
width: auto;
display: table;
float: none;
margin: 0 auto;
}

The JS window should provide a list of styling classes to chose from (that is, to style the container and not only the image).

Have a look at http://drupal.org/project/wysiwyg_imageupload

That module has the desired feature.