You can right click on the Timeline anywhere between the two slides that you want to maintain the positions of and "Insert a Blank Slide" as many times as you want without moving the previous or next slides. You can add you're images in O&A or by right click on the slide and "Change Image File".
It does not LOCK the position of any slides but goes part way toward what you want to do.
Phil
P.S. The positions of slides added this way can be adjusted by placing the mouse cursor at the beginning of the slide until the double headed arrow appears and dragging in the Timeline. This also does not move the position of the next and previous slides.