The majority of images available for download on the Internet are 72 ppi (pixels per inch). Why is this?

An image which has 72 ppi has enough information to display very well on a monitor, yet the file size isn't too large. If you are searching the web for images to display on a computer monitor (projected with PowerPoint or Keynote, for example), then an image which is 72 ppi will suffice. You then only need to consider pixel dimensions. To understand more about what size of images you need for your presentations, see our Best Practices for projection page.