Placeholder images are helpful for development mockups and demo apps because it gives the viewers an understanding of what the page could look like in a real layout or even in deployment (if the placeholder is an image and not just a dimensions box). There are different kinds of placeholder images with varying levels of advantages.
images, cute
offline, javascript, dimensions
This is a great one for sketch-level mockups because it’s javascript and can be used offline.
However, it is a bit more limited because it just displays the placeholder and the dimensions, it does not display pictures, though you can change the colors.
pictures, dimensions