Easier way to get Lightbox for images?

I like the lightbox effect for the featured image on a post.

When I want to allow the user to enlarge other images on the page, I can activate the lightbox by adding the code:

<a href="url" class="td-featured-img">

Is there an easier way to do this without adding that class into all of my images when I create posts?

Thanks in advance for any help anyone can provide!

hi,

for the moment your solution is the best solution,
another one is to search for a plugin that duos this.

Thanks for you message.
Radu A.