How to undo the "Place one picture on top of other"

[JannyM.]

Trying to fade out one picture as the other fades in by reviewing everyone's
questions in the group. Very helpful I might add for quite a newbie as
myself. However, apparently this instruction worked so well that I have no
clue how I did it. It seems I placed the wrong picture on top of another
picture and though I've been trying to find indication of the each pic within
each other so that I may undo my mistake, I am having no such luck. How do I
undo this? Also, if you can instruct me on how to undo this and also how to
do it the right way I would greatly appreciate it. However, you have to be
verrrrry basic, since I do not even how to "Place one picture on top of
[an]other".
~JannyM.