One of the key factors to whether your digital picture looks good when it is printed is its resolution. Resolution simply means how many pixels, in other words how many dots, are in your digital photo. If the resolution is very high (i.e., there are many dots), the picture will look crisp and sharp. If the resolution is low, it will look jagged and rough, especially at large print sizes.
All things being equal, a digital camera with more pixels will produce pictures with better resolution than a camera with less pixels. A 1 mega pixel camera means that the camera records 1 million pixels or dots per picture on its highest quality setting. Be aware that if you change your camera's settings to allow it to hold more pictures on the memory device, you may be reducing the number of pixels per picture, thereby affecting the quality when you print your photos.
The better the resolution, the larger the file size generally is. Larger files can clog email boxes and take a longer time to download from a web site. As a result, many pictures you see in emails or on non photo-related web sites are often scaled down to a low resolution that is not suitable for high quality prints.
If you are using an online photo ordering system, it will tell you if a picture does not meet there suggested resolution standards. When you add an image to our shopping cart, it will tell you if the image is OK to print at that size or not. If the quality is going to be questionable, you can get more details about why the resolution is less than what they suggest.