When accessing the Add images section, you will be able to drag & drop images from your computer.
The name of the images that you have dragged will show up, you can remove one or several of those images and you can finally click on Start Upload.
The progress bar will show up and you should see a screen like the one above. This tells you that your images are being added to the platform.
After adding images via drag & drop, you can find those images in the Not tagged yet tab. In that tab, without any other sorting, the images are sorted by batch. You will then find the images you've just uploaded on the last page of the section. However, within a given batch, the order of the images will not be exactly the same as the order in which you dragged the files, or the order of your images in the JSON file.