Image resolution is something that should always be checked before sending any artwork to be printed.

Resolution

Resolution simply means the quality of an image. A digital photograph is made up of millions of tiny dots; the more dots you have, the better quality the photo.
The ratio used to measure images is dots per inch, or dpi for short. So an image which is 600dpi (600 dots per inch) is going to be higher quality than an image which is only 150dpi.

Computer screens can display images perfectly at just 72dpi. Most images you see on the internet are set to 72dpi, as it keeps the file size small and there is no need for their resolution to be any higher. Printed images must be a resolution of 300dpi or greater to print properly. It is important to double check the resolution of all images before sending them to a printer, as a 72dpi image can appear perfect on screen, but look fuzzy and badly focussed when printed. A low resolution image which has been printed is often referred to as being ‘pixelated’.

As a general rule it is preferable to start with images that are 300dpi or greater to begin with. After all, you can throw away pixels to make a big image smaller, but adding random pixels to make a small image bigger is not recommended. If you are have a low resolution 72dpi image and must get it printed, there are a couple of tricks to increase its resolution up to 300dpi.

The first method is to squeeze the dots closer together. This has the effect of making the image smaller. For example, a 50mm square photo at 150dpi would become 25 mm square photo at 300dpi. In other words, the image resolution has doubled (150dpi up 300dpi) while the image size has halved (50mm down to 25mm). This method is fine if you can work with a smaller photo, since the image quality is not degraded in any way.

The second method, which we do not recommend, is to keep the photo at the same size (50mm square) but increase the resolution up to 300dpi. This is done by ‘resampling up’, which means the computer adds pixels into the image that weren’t there before.

Unfortunately the problem here is the computer isn’t human. It doesn’t know what the overall image is supposed to look like. If it sees a blue pixel, it adds a few more blue pixels around it. The same goes for every other pixel in the image. The end result is a 50mm square photo which has lost the fine detail and contains visibly messy streaks of colour.
This is why adding pixels into an image to increase its resolution is not recommended.

You could possibly use this technique if the image were a simple one (does not contain complex details) and if you only needed to make a small resolution adjustment (nudging 270-280dpi up to 300dpi.) However in almost all cases it would be far better to either find a higher resolution version of the same image, or increase the resolution by shrinking the size.

There are a number of image file types, but the main three used in print are JPEG, TIFF and PSD. You can tell what file type an image is by looking at its file name extension – .jpg, .tif or .psd.
PSD is a format which allows multiple layering and advanced effects, resulting in very large file sizes. JPEG is a ‘lossy’ format, meaning pixels are thrown away when the image is compressed. TIFF is a ‘lossless’ format; better quality than JPEG, but the file sizes tend to be larger.

You might also come across another file type called EPS (file type .eps). This file type is used for vector images such as logos and illustrations. Vector images are resolution independent, meaning they are not comprised of pixels.