Sorry at present, it is not possible to have multiple iphone images for slider shortcode. You will need to add a shortcode of your own for additional images or you can use one of these shortcodes –
galaxys4_slider, htcone_slider, ipad_slider
and change the images for these sliders to change to the black iphone one after changing the image file names for these sliders. Of course, you will need to adjust the css a bit (see below) so that the content aligns with the phone image (problem you are facing currently).
To fix the alignment, I will need the link to the page. You can also try tweaking the top, left, height and width values and see if they make a difference (put the modified code in Custom CSS tab of theme options) –
.iphone-slider-container .flex-slider-container {
top: 16.3%;
left: 16.8%;
width: 63.5%;
height: 62%;
}