The placement of the cans depends on how dramatic a look you want. If you place it behind the palm, the leaves should make a shadow on the wall for a more dramatic effect. Cans are normally placed behind something so that you aren't looking at the light as you pass by.

BUT you need to be careful if lighting a live tree. Sometimes the light will cause problems. It may be necessary to use a true daylight bulb in the can. Also, depending on the wattage the can is rated for, you can put a smaller bulb in the can to still give the effect but not harm the plant.

As for other placement of the cans, they are great to play with. Again, you want to be careful not to have too high a wattage lamp that may cause material to heat up. Some people have placed them in the corners of a room for an added dramatic touch.

Play with them and see where you like them.