Image size in pixel. why?
The question you asked is quite technical and I hope my answer is sufficient.
Pixels are small squares of digital information with color values. Pixel dimensions are the measurements of an image expressed in pixel. Physical dimension is the measurement done in cm. or m. etc.
Images of different physical dimension might have the same pixel dimensions. This is because these images will have different resolutions. An 8×10 image with 150ppi resolution will have a pixel dimension of 1200×1500. Again an 4×5 image of 300ppi resolution will have a pixel dimension of 1200×1500.
As you may know an image with a higher resolution can be seen more clearly with the naked eye. If I give you the physical dimensions of an image rather than the pixel dimension, you will have no I idea of its resolution. But if I mention the pixel dimension than you don’t need the resolution because it will obviously be related to its physical dimension (greater the physical dimension, lower the resolution). This is why images and videos are expressed in pixels.