For each new image you upload, you'll need to create a new object (of type System.Image or a specialization of this entity) for the image to be stored in. For instance, if your images were displayed in a Datagrid, the New button on the datagrid would create an image object each time it is clicked. In this way you could create and upload 20 images.
I am not sure how you've built this currently, but you could look at the DropZone widget in the appstore, which provides the ability to upload multiple images.
Thanks Mike, I've got it working. The solution is displaying the images in a Datagrid and via the New-button create a new image object