Add an image by clicking 'Add image(s)'. You'll then get an overview of all uploaded images. Here you can select a previously uploaded image or upload a new image.
Add a description
The images description can be shown under the images and in the larger view of the gallery. A description also helps make your website more search engine friendly as it allows a search engine to 'see' what is on the image.
To add a description, move your mouse over the image like in the animation below and click 'Edit description'. You can then enter a description, in multiple languages if relevant.