Hi Gwen,
I'm not sure that I understand the effect that you have in mind.
At the beginning of the slide, is the picture supposed to pop up from under the floor? or is it simply lying on the floor?
In any case, if you do require a mask (and I'm not sure that you do) - then you need two duplicates of the same picture. One duplicate appears from under the mask and reaches a certain point on screen, and then stops (no more keyframes for it). The other duplicate continues the rest of the motion, for all the remaining keyframes, that are on top of the mask.
The tricky part is to find that precise point where the switch between these two identical layers is not visible.
You may have a look at a similar effect that I made: Photo Table
http://seemonasblog.blogspot.co.il/2013 ... tyles.htmlThese effects were created without any mask.