Not completely sure I understand what you mean, but I'll give it a shot.

Do you know how to use guides? Open your image(s) and press ctrl+r to add a ruler to your picture. Hold the mouse over the ruler on the top of the picture, and click and drag the mouse down. When you let go of the mouse button, a guide should be made.

Make guides for the top and bottom of one picture. Bring in the other picture and use ctrl+t to scale it so both pictures match up to the guides, making them equal height.