To add image slides, hover the Add Slides button and select Image slides . The WordPress Media Loader will be opened, where you can select a single image or multiple images (by holding the CTRL key on PC and Cmd key on Mac).
Note: When you select an image, also select the most appropriate size for it in the Media Library's right sidebar, under ATTACHMENT DISPLAY SETTINGS . You will want to select an image size that is similar to the slide size.
After you select the images, click on the Insert into post button, and one slide for each selected image will be added in the slider.