Image resolution and associated abbreviations such as 'dpi' (dots per inch) often confuse a lot of people. This page explains what resolution is and how it needs to be taken into account when creating layouts or printing pictures.
The resolution of the original image is the critical factor. People sometimes make the mistake of thinking they can take a low resolution image, put it in Photoshop, and then change the resolution to 300dpi to make it a high resolution image.
While Photoshop can usually effectively double the resolution of an image using interpolation, that's about the limit. Changing a low resolution pixelated original image in Photoshop to 300dpi will just result in a very large file size 300dpi image that is still pixelated. You need to start with a high resolution original, and you also need to create your original Photoshop files in high resolution.
When an image is sent to a certain output device, such as a printer or a monitor, its resolution becomes important because each device or medium needs a certain base resolution to reproduce the image with the best possible quality. See our handy guide below for more.