Three way switch
You will need to tie your new light fixture into either the other switch box -OR- the existing light fixture. The switch box at the top of the steps does not seem to have a neutral (grounded conductor) in it. It is hard to say for sure without being there, but it probably would be best to tie in at the existing light fixture, assuming you can reach it and you don't exceed wire fill capacity of the boxes involved.
I hope this helps,