To find the posters got to the object window and, make sure that all is selected at the bottom of the list, then at the top of the window type "poster" into the filter box. You'll then get all items that have the word "Poster" in the name, this will find most posters but, not all.
Next got to the world menu, open Object editing pallet. A window will pop up, if you haven't used this feature before the box will be empty. Select all the posters in the "Items window" and, drag them over to the new "object editing pallet" window. You will now have a list of all posters that you can scroll through and see which ones you want to use.
Save your new object editing pallet by, clicking the save box in the Object editing pallet window! This feature of the GECK will become your best friend.
Placing your new posters:
In the Object editing pallet, select the poster you want to use. In the window you'll see all kinds of placement data referenced by x,y,z boxes, don't worry about this until you get more familiarized with how to use the pallets.
For now all you need to look at is the "Conform to Slope" Check box and, the "Sink" box.
First, check "Conform to slope",
next set the "Sink" to -1,
Finally Under the window, click the "Apply settings to preview" button.
Now your ready, with your pallet set up you can now Control, option, Click and the object will be placed in the render window. You'll have to rotate some of the posters as they will come in at 90 degree angels. Use the 45 degree button and, turn them against the wall.
Remember, they don't need to be completely flush with the wall, if they are floating a few point away from the wall you'll never notice it in the game. Just make sure they aren't too fare far away or, too close.
For more info on using the Object editing pallets look here, http://geck.gamesas.com/index.php/Object_palette
Hope this helps,
cev