Resolution refer to the amount of detail an image contains. It is describe as Dots Per Inch (DPI) on the computer screen. The more DPI an image has, the more details it contains. If your file has a low resolution, it means that there are fewer dots or pixels contained within an inch. Therefore it will look blurry of jagged on screen and on print.
For products products of size A3 and smaller, your file needs to have at least around 300dpi, so that the resulting print appears crisp and clear.
While for products above A3 size, like posters, banners and flags, 150dpi will be good enough as these products are seen from afar.
The example below shows the contrast between High and Low resolution.
To check the resolution of the images using Adobe Photoshop
Navigate to: Image > Image Size.
If the resolution that you see here is lower than 300dpi, you should start a new file according to the steps that we outlined in “Setting up your file”. If you simply change the resolution to 300dpi, the existing image will be stretched and not have the desired outcome. As there is no way to turn a low-resolution image into a high-resolution image.
To check the resolution of the images using Adobe Illustrator
Navigate to: Window > Document Info
Ensure that the resolution is at least 300 x 300 pixels per inch.
If your graphics are of lower than optimal resolution, you may want to request a higher resolution graphics from your designer or your content source.