What Image Resolution ?

When you’re working with a bitmap image, image resolution is key. The image resolution is the number of pixels along the width and height of an image, commonly measured in pixels per inch (ppi). For example, a common image resolution is 300 ppi, which means the image is 300
pixels wide and 300 pixels high, for a total of 90,000 pixels of data.      Because each pixel contains specific information in a bitmap image,it’s good practice to think ahead and work with images that are in the resolution in which you’ll ultimately want to print or publish. Generally, though, you’re better off starting with a higher resolution (which means you have more data) and scaling down than you are starting with a lower resolution and scaling up, which would require Photoshop to manufacture new pixels.