Allow on* JS handlers for IMG elements in WordPress / TinyMCE 4

gistfile1.txt

add_filter( 'tiny_mce_before_init', 'my_mce_init', 20 );
function my_mce_init( $init ) {
	if ( current_user_can('unfiltered_html') ) {
		if ( ! empty( $init['extended_valid_elements'] ) ) {
			$init['extended_valid_elements'] .= ',';
		} else {
			$init['extended_valid_elements'] = '';
		}

		$init['extended_valid_elements'] .= 'img[id|accesskey|class|dir|lang|style|tabindex|title|contenteditable|contextmenu|draggable|dropzone|hidden|spellcheck|translate|src|alt=|usemap|ismap|width|height|name|longdesc|align|border|hspace|vspace|crossorigin|onclick|ondblclick|onmousedown|onmouseenter|onmouseleave|onmousemove|onmouseover|onmouseout|onmouseup|onkeydown|onkeypress|onkeyup]';
	}

	return $init;
}