Since almost all readily available monitors start with 120 volts, the most practical way to power the pixframe is to run 120 volt power, ideally through the wall, to a disconnect plug so that the monitor can easily be removed from the wall.
The plug that is connected to the pixframe has a wire that is soldered to two leads:
one pair is soldered to the monitor power cord; the other is soldered to a 5 volt raspberry pi power supply.
I crack it open, remove the flat wall prongs and solder a connectoin to the incoming power and close up the hole with epoxy.
I like to run the power behind the wallboard: I drill a 5/8" hole in the wall behind the pixframe and a corresponding hole just above the baseboard directly below. I drop a light chain down the hole and find it with a magnet. This is used to pull a length of zip cord. I put plastic wiremold along the baseboard to cover the bottom hole and conceal the wire until it exits near a wall socked behind a piece of furniture where I can put a power strip on the floor.