The amount of dots per inch (or on your monitor) pixels per inch displayed, decides the sharpness of your image.
If the image comes from a camera, the resolution the photo was set to take will decide the resolution. If it was created on a computer, then the resolution you set when starting out, determines it.
Simply up sampling, or scaling up, the picture within Photoshop will not correct poor resolution which can result in pixellated images.
Make sure you do not reduce files sent to us when using your email program, or by creating your template at the wrong resolution. 300 dots per inch is the minimum suggested resolution for print, while we can reproduce more, at higher resolutions you are better to use vectors for your text.