Pixels are the actual image grid
A 2400 × 1600 image contains 3.84 million pixels regardless of whether metadata labels it 72 or 300 pixels per inch. For screens, CSS layout and the device's pixel density determine how large and sharp it appears.
For print, physical size and pixels per inch work together. At 300 pixels per inch, 2400 pixels span 8 inches; changing the PPI tag alone changes the nominal print size, not the underlying pixel count.
Why dimensions matter online
Browsers download and decode pixel data. An image with excessive dimensions can cost extra bytes and memory even if CSS scales it down visually.
Prepare dimensions for the intended display and use responsive sources when page layouts vary. Don't confuse changing a PPI metadata field with resizing the pixel grid.
Frequently asked questions
Does changing DPI reduce file size?
Changing only resolution metadata usually does not reduce the pixel data or meaningfully shrink an image.