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.