Keynote - Align pictures across multiple slides

I'm not sure the best way to do it, but I often do this as follows:

1. Create Slide 1 and Slide 2 with their respective pictures 1 and 2. Get Picture 1 where you want it in Slide 1.
2. In Slide 2, cut Picture 2.
3. Move back to slide 1 and paste in Picture 2. Align it right on top of the picture in slide 1; it'll snap into place nicely. If they're different sizes, you can adjust that too.
4. Finally, cut Picture 2 out of Slide 1, move to Slide 2, and press paste. It'll stay in the same position.

A second technique would be to manually copy the relevant attributes (size and position) from the Format > Arrange panel.

I think you could also use guide lines.