Image size and resolution are two terms that are often confused. Although both linked to images, they mean two different things.
Image size is a count of the number of pixels present in an image. This cannot be counted manually, but must be done by a computer as pixels are usually too small to see with the naked eye.
The computer counts the number of pixels in the height and width of an image, and multiplies these numbers together to produce a measure called the image size. This is done in a similar fashion to the way that the area of a rectangle is calculated.
Resolution, however, cannot tell us how big or small an image is. It is a measure of the quality of the image by counting the number of “information dots” in a square inch of the photograph or image. Naturally, resolution is spoken about in terms of DPI, which is short of dots per inch.
It is important to note that in most cases, print resolution is substantially higher than screen resolution. Because most computers have a resolution of 72dpi, that is the same resolution that should be used for images displayed online.
Resolution is a key factor when it comes to the properties of an image, because it also directly impacts the speed at which it can be displayed on a screen. Amazingly, high resolution images may not look any different to normal resolution images. This would mean that a higher resolution for online media is completely unjustified.
In light of this, it is greatly advisable that images are compressed before online publication. Use www.compressimage.org for all your photo compression needs. This online image compression software is one of the best on the market, and absolutely free to use.