I bought 5 meters of the Adafruit 30 LED strips, or 150 pixels.
The poster frames would hold 10 pixels in a column, so I cut the neopixels into 10 LED strips. This left me with 15 strips, but since I wanted it to be even I used 7 on each poster and kept the extra 15th strip.
I divided the poster into even rows of 7 by 10 using a square ruler and then superglued the neopixel strips to the inside back of the poster. I alternated direction, snaking them up and down, to save on resistance in the whole circuit. I compensated for this with the programming. Then, I soldered the power, ground, and data lines back together to make one long strip again. I drilled holes in the back of the poster to stick the beginning and ends of the neopixel strips out of the back of the frame, and used JST connectors to connect the two frames together. Then I used a glue gun to reinforce all of the connections.