To add an image gallery, you can use the built-in WordPress gallery option. Simply click the “Add Media” button at the top of the post content and select the “Create Gallery” option. Then simply select the photos you want to use and click the “Create New Gallery” button. However, before you finish and insert, be sure to check the number of columns and where the images are linked (to the Attachments page if you're linking to a media file for a lightbox of images, or if you want to open the image in a new tab or custom URL).
The second option is to use any gallery or media options included in the theme. Some themes include a gallery post format, and when you create a post, you'll see a “Gallery” meta box below the content. Simply use this option to “Add Image” to select and insert the image you want to use in your gallery.
