Before you attempt to change the DPI of your images, it helps to understand what DPI means and what it does. DPI stands for Dots Per Inch. When an image is printed on a printer DPI is used to determine how big the image will appear on the printed page.
The printer will use the DPI setting of an image file to calculate how many ink dots it will print for each group of pixels in the image per inch of print. Generally, the more DPI, the higher the quality of the print (more sharpness and detail) and the smaller the print will be. Changing the DPI of an image does not change the underlying quality of the source image, only the size (and quality) of an image when it is printed.
The photographs produced at Blink Inc are all full-size high resolution JPG images. Our images as delivered to you are 5184 × 3456 pixels in size, and will print from 4” x 6” up to 30” x 60” at 300 DPI. We set the DPI of each image to 72 DPI because this is the most common setting for the images to display correctly on screens and printers without any issues. If you are printing on a high-resolution printer, you (or the printer you hire) must change the DPI of the images you are printing to the proper DPI for the size and quality of the final print you will want.
A high-quality printing service will be able to change the DPI of the images they are printing to the appropriate setting for the size of print you want and for the printer they use. You should not attempt to change the DPI of images without a thorough understanding of how your image will be printed, as an incorrect setting for DPI can cause image quality issues when the image is printed.
For a really thorough tutorial on image printing and DPI, we recommend the following links:
What is DPI / PPI?
How to set DPI using Photoshop