There are 47 paste-ups and can be obtained any time. For it to be easier use the construction worker which you get in an early mission story and use the cargo drone to get to the location easier.
Edit: Confirmed by Wookiee Hairem that it doesn't need to be the same image.
Be sure to use the same character when going for them
and I think this is the most important step. Use the same image for every paste-up! I'm not sure this makes the achievement 100% pop but this is what worked for me when it didn't unlock the first time I went for it.
For the locations this video showcases very well every single one:
(It's my first guide so any criticism is well appreciated)