Good point to bring up...
Images on the web are measured in pixels. Viewports on the web, all CSS, etc, everything is by Pixel. Even when setting a size in EM, EX, REM, it's converted to pixel. Yet many people go through the trouble of setting their images to 72 dots per inch ( DPI ). The process of sizing images for the web is often misunderstood.
Sorry, I'm a Web Developer. xD