The number of pixels in an image refers to its size, meaning the number of pixels present within the full length or width of the image. Pixels typically do not affect the display size of an image on a website, but rather the quality of the image when printed.
Pixels are a common unit of measurement for computer images, because a pixel is a single dot within the display of a computer screen. When an image appears in a website, the browser reads the number of pixels within the images, as well as the color information for each pixel, and displays it accordingly on the monitor. If the site includes code for increasing the scale of the image beyond its native dimensions, it may appear more blurry and unclear. However, this artificial display size does not affect the way the image looks during printing, as that involves its resolution, which is the number of pixels that appear within the physical image per inch.
Some devices do feature a special type of display that offers a higher resolution for all graphics through a process that involves compressing the image to display twice the number of pixels within the same space. These devices, such as Apple's line of Retina Display phones and laptops, may display some images blurry due to the pixel compression technique.