An image is made up of tiny dots called pixels. Each pixel has a color that is a combination of red, green, and blue. To save each of these pixels, we use 3 bytes or 24 ones and zeros. If the image is big, it could have millions of pixels, which means it will take millions of bytes to save it on a device.
When a camera or cellphone has 10 megapixels, it means that each photo has 10 million pixels. That means each photo takes up 30 million bytes or 30 megabytes of space! If you want to send this photo or several photos to a friend by email, it will take a while to upload and a lot for your friend to download it later.