You need to have knowledge of the UV arrangement used with the content was made. Without it, any image you try to place in Poser's Material Room will either show up the wrong size or distorted because of misplacement on the mesh. Check if the vendor provided a UV template for all the content in his/her product, usually it will be a .jpg image which can easily be opened in Photoshop or GIMP. Open the template in your 2D software, then add your custom image to another layer. Create any additional masks based on the same template and save the necessary images. When you return to Poser, you can add the newly created patchwork to your content. Again, this is only possible if UV templates exists. Otherwise, having to create UVs from scratch is a rather long, complicated method involving UV Mapping software and a bit of experience.