I did kinda exactly this... let me make some screenshots.
Lets say I use a picture of the Niagara-Falls and want to give it a new texture so I can put on another image
next I import the texture and gives it the size of the actual poster (which is 157cm width and nearly the 49,95cm it says (actual its 53cm height, but thats due that this is a image))
now I change the object to match my needs
after what you have said it should work now, but look yourself:
but the image looks like this
So you see there is a point in the object where it tiles the texture around it.
