Watermarking in Photoshop

[mbalensiefer]

Hello!

 I have a .psd image that I'd like to position as a watermark in the same area within each of my pictures.
I think I am looking for something that measures out a percentage-distance from the sides, and then the top and bottom...like an "insert in."

My pictures are of different sizes but of the same dimensions.

Thank you!
V/R
Michael