Depending on what version of PSE you have, you could do the following:
1. Copy your photo to a new layer above the shape you want it to assume.
2. Right click on the photo layer and choose "create clipping mask" or "group with previous". Your photo will then be clipped to the shape of the layer below.
3. At this point, you can select your photo layer and move or resize it so it fits how you want in the shape.
4. After you have it how you want it, lock these two layers together.
5. If you are sure you won't want to make any more changes, right click on the photo layer and choose "merge down". The layers are then merged together and only your shaped photo will show.
I hope that all makes sense. Here is a tutorial from Holly with more detail: