Open a presentation, click sidebar, then click an image thumbnail. Enter your text in the Image Description text box. The image description isn’t visible anywhere in your presentation.
You can also add a title to an image gallery, as well as add captions to individual images or all images. See sidebar, click the Gallery tab, click the image thumbnail, then press Delete. To extract an image from the gallery, drag the image thumbnail to a slide, document, or anywhere you want to place it.