Liviu,

1 - Give the image you want to use the exact size it should have, say 60x90 cms, using The Gimp or similar graphics program.
2 - Now import that image as a texture and give it those measures during import.
3 - Insert a box and give it those measures, then apply the texture.
4 - If the image isn't visible, exchange width and height of the box, so that the image becomes visible.

Hans