JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdn.ckeditor.com/4.5.7/standard/ckeditor.js"></script>
<textarea name="editor1">
</textarea>

JavaScript

// Set the callback function 
var setLandscapeClass = {
    exec: function(editor) {
        editor.getSelection().getStartElement().addClass('ckLandscape')
    }
}

//Create the plugin
CKEDITOR.plugins.add('setLandscapeClass', {
    init: function(editor) {
        editor.addCommand('setLandscapeClass', setLandscapeClass);
        editor.ui.addButton("setLandscapeClass", {
            label: 'Set Landscape Class', 
            icon: '',
            command: 'setLandscapeClass'
        });
    }
}); 

// Create the instance and add the plugin
CKEDITOR.replace( 'editor1', {
    extraPlugins: 'setLandscapeClass',
    allowedContent: true
});

// enable/disable the button based on the selection of the text in the editor
CKEDITOR.instances.editor1.on( 'selectionChange', function( evt ) {
    var landscapeButton = this.getCommand( 'setLandscapeClass' )
    if ( evt.data.path.lastElement.is( 'img' ) ) { 
        landscapeButton.enable();
    } else {
        landscapeButton.disable();
    }
});