To add an image to the FFS Group,, when uploading you can select down in the left hand corner the option to make your image private . Untick the public box and tick the "only me" box. (Private images always have a little pink box instead of green attached to the image...)
Images are visible to anyone in a group to which a private image is added to. This also means your FFS image is invisible on your photostream while visible in the FFS group.