Wrap the Dropzone widget in Mendix. This allows users to drag and drop images in the browser. Pretty intuitive.
With the standard file manager this does not seem possible. What you could do is let the users upload a zip file with the 7 image files. Then with the help of a little java unzip the uploaded zip files and create the image documents from the zip and check if there are at least 7 files. To make sure that the files contained in the zip are actual images another piece of java can be used javax.activation.MimetypesFileTypeMap. With this you can check the mime type of the file and see whether this is actually an image.
Hope this helps you in finding an easier solution for your users.