I've done similar things in the past although not with rose petals. What I would do is create a series of png files. The layer closest to the camera would have the most amount of the rose erased. Then each successive picture would add a petal. Then you place your pictures in between each layer and scale them to fit the way you need. The last picture in the series would serve as a background so it would be a complete rose.
Here's an example of how I would do it. The pictures are not to scale but are there just to show the placement possibilities.
There are no doubt other ways to do this, perhaps even completely from within ProShow, but I find this way to be more accurate and easier to work with. In this case I used Photoshop to create the separate layers but any graphics editor should be capable of doing it.
Joe