Image resolution is the detail an image holds and is frequently meaured in dots per inch (DPI). Higher resolution means more image detail IF the image size is the same. For example a 4"x6" image at 72 DPI has less resolution than a 4"x6" image at 300 DPI. The 72 DPI image would be fine for a website or PowerPoint presentation. However, it would not be as sharp or clear when printed as a 300 DPI image.
As is true with most things, it's not as simple as that though. When selecting images for print, you need to focus on both RESOLUTION (DPI) and IMAGE SIZE. For example a 300 DPI image at 4"x6" would not be sufficient for a catalog cover, banner or any printed item of any size.
Low resolution images are used for websites and other web applications. These images are usually 72-96 DPI (dots per square inch) and are very small in size. The lower resolution makes these images low quality, and they are unable to be sized up for print and maintain image integrity. If an image looks great on your computer screen that does not mean that it will look good when printed.
Most images taken with a cell phone will not be of print quality resolution, so be sure to check your quality and size. Newer phones take better pictures, but cameras do the best job.
High resolution images are used in print media, such as posters, business cards, brochures and other signage. Print quality images range from 150-300+ DPI and are much larger in size. A non-vector (.ai, .eps) logo file that is of print size should be at least several megabytes in size. The bigger the better for print files.
So if you use a 300kb file for a 24" x 18" poster, you will end up with a great big blur! And nobody wants that! You want to be able to see your image. If you have had a designer create your logo or other aspect of your design, please contact him or her for a high resolution version. This will typically include an .eps, .ai, .pdf or .psd file. Remember: images or logos created in Microsoft Word or PowerPoint will NOT produce high resolution files.
Ok, if all of the above has left your head swimming, let's go with a sandbox analogy.
DPI = Depth of Sand
Pixel = Grain of Sand
Image Size = Size of Sandbox
Imagine a 4' x 4' sandbox with 3" of sand (like 300dpi) spread evenly across the sandbox. If you expand the size of the sandbox to 5' x 5', there is still enough sand to cover the bottom of the sandbox. However, if the sandbox expands to 20' x 20' and the sand is spread out evenly, there is not enough sand to cover all of the bottom of the box. That is equivalent to using design software to exapand an image for use in a larger format. At some point there are not enough pixels to keep the image sharp. The bigger you make the image, the blurier it gets.