Don't change any of your image files! If you are going to have a lab make the images, just take them in on a CD or memory card or USB Flash Drive and tell them what size images you want from each. If you eventually get a photo printer at home, then in Photoshop select the "Print with Preview" option and set up your page, size of paper and size of print and you will get what you want. You may have to play with the sizes, but you will understand how to print in a short time. Be sure to select photo paper when doing the set up, because the default is "plain paper/bright white paper".