replacing an image with another in photoshop

[Savannah5FED]

i am trying to replace a silouette of a girl with an image of the stars but it wont let me select the girl. could some one help or advise some different tools to use

[Norman Sanders]

It would be helpful if you posted the image and Layers panel

[Savannah5FED]

 these are the photos 

 this is my layers panel

[davescm]

To select the girl either select that layer in the layers panel, before making your selection within it,  or turn on "Sample all layers" in the options bar for the quick selection tool

Dave

[Norman Sanders]

Calculations commnd set as shown will produce the left image. Use the Lasso tool and Edit > Fill to clean up the white areas and finish by using the Brush tool > Options bar: Ovetlay and set to White and then to Black to finish the silhouette. (It does an excellent job on the hand.) Work with an upsized view of the image.