When inserting images into post or page content WordPress automatically adds three class name to the image tag along with the other attributes, these are the alignment (alignnone), size (size-full) and the attachment id (wp-image-123).

If you need to add additional class names to the defaults without having to manually add them every time you insert a new image, you can make use of either the get_image_tag_class or get_image_tag filters. Continue reading →