My previous instructions should have worked for you.
What version of Photoshop are you using?
Once you have moved one image on to the other you should have two layers showing on your Layers Palette.
Select the top most layer on the Layers Palette (this should be highlighted blue to show it's active).
On the Layers Palette where it says Opacity 100% - change this to about 40% - 30%.
You could also change the Blending Mode next to the Opacity from Normal to Screen or Multiply.
The only other thing I can think of to help you is check your images are both the same format i.e. JPEG or TIFF.
Also check the Image Size > Resolution. They should be the same and set in Pixels Per Inch. 300 ppi if you are going to print the pictures out.
If none of the above work for you try a Google search for a tutorial on blending images using Photoshop.
I've listed one to get you started.
Good luck Scarlett.
http://www.picturecorrect.com/photoshoptips/bl ending_photos.htm