CKEDITOR.on('dialogDefinition', function (ev) {
    var dialogName = ev.data.name;
    var dialogDefinition = ev.data.definition;
    var dialog = dialogDefinition.dialog;
    var editor = ev.editor;

    if (dialogName == 'image') {
        dialogDefinition.onOk = function (e) {
            var imageSrcUrl = e.sender.originalElement.$.src;
            var width = e.sender.originalElement.$.width;
            var height = e.sender.originalElement.$.height;
            var imgHtml = CKEDITOR.dom.element.createFromHtml('<img src="' + imageSrcUrl + '?width=' + width + '&height=' + height + '" alt="" style="width:' + width + 'px;height:' + height + 'px;" />');
            editor.insertElement(imgHtml);
        };
    }
});

In CKFinder these variable are easily changed with the following code:

$config['Images'] = Array(
'maxWidth' => 1600,
'maxHeight' => 1200,
'quality' => 80);

Here's a solution (finally):

Add the following to your config.js:

CKEDITOR.on('instanceReady', function (ev) {
    var editor = ev.editor,
        dataProcessor = editor.dataProcessor,
        htmlFilter = dataProcessor && dataProcessor.htmlFilter;

    htmlFilter.addRules( {
        elements : {
            $ : function( element ) {
                // Output dimensions of images as width and height attributes on src
                if ( element.name == 'img' ) {
                    var style = element.attributes.style;
                    if (style) {
                        // Get the width from the style.
                        var match = /(?:^|\s)width\s*:\s*(\d+)px/i.exec( style ),
                            width = match && match[1];

                        // Get the height from the style.
                        match = /(?:^|\s)height\s*:\s*(\d+)px/i.exec( style );
                        var height = match && match[1];

                        var imgsrc = element.attributes.src + "?width=" + width + "&height=" + height;

                        element.attributes.src = imgsrc;
                        element.attributes['data-cke-saved-src'] = imgsrc;
                    }
                }
            }
        }
    });
});

This code is run whenever the CKEditor generates the actual HTML, which happens when you either view the source by clicking the "Source" button, or by performing an HTTP POST of that page.

A small warning, though. The code above will keep appending the width and height query strings for each click on the "Source" button, or for each postback, so you might want to add some extra logic to filter out the width and height query strings before appending them to the src attribute.