Add a style definition as described in the How do I customize the Styles drop-down list? article and pass the class name in the attributes parameter.

Do remember that since class is a resereved word in JavaScript, you need to place it in quotes.

The following example adds a myClass class to an img element. The image element will now be styled as defined in this CSS class.

{
	name : 'Custom Image',
	element : 'img',
	attributes : { 'class' : 'myClass' }
}

For more details on the definition format and best practices on how to customize the styles please refer to the Styles article from the Developer's Guide.