So should I find a custom frame in that shape or will a long rectangular photo work as well?

I agree with the above comment. A nice picture when you walk in would be nice. I would also paint the walls a lively color and add a little table and a plant or two. Maybe a chair in the corner because sometimes when you get off from work, all you wanna do is sit down as soon as you walk through the door.

I think you would have to play with the shape of the picture frame you want to add. It may even look nice with multiple pictures in it.

Corinne, you can be bolder here. Blow up to cover the whole interior - your molding is already providing a beautiful frame.

How about trying a nice wallpaper