I would not recommend creating the entire poster in Poser. Poser is specifically for character modeling. So it would be helpful to create the male model in poser and then export him into a JPG file or something you can work with in Photoshop. If you’ve never worked with 3D software, I would really not recommend using it for the first time to create something for the professional work space (like a print ad). A simple graphics or photo editing software will be much more forgiving and less time consuming.
However, should you decide to pursue the project in Poser or another 3D program… you can create the building and text in photoshop and then pull in the male model to create what you need.
You can also check out iStockPhoto.com for a lot of really great high-end graphics. Doing it this way will be a bit pricey, but it’ll save time. Good luck!!