Hi Queen_of_herts welcome to Rising world hope your having as much fun with the game as we all are, for your posters here is how you get them working.
Make sure you have the poster selected in 1 of your slots, left click and this will bring up the poster menu on the left hand side click on the button that says "upload image", find the image that you want to use ( a preview will show up on the right hand side) and click on "Select image" then click on "Use image" you will now be able to see the image in your game.
Don't forget you can now rotate the image with the arrow keys and also holding down the left shift key will let you resize the image as well.
Also make sure that these are in jpg / png format (not to sure about any others) .
One last thing to keep an eye on the updated version click on the Roadmap at the top or here https://trello.com/b/t5Leypcj/rising-world-development or of course just check back here on the forum.