Images are composed by several dots called pixels, and each of them has a color, represented as a combination of three basic colors (red, green and blue). To store each of these pixels, a byte (or more) is used, and that means storing all information for an image file that is in a computer or any device will take millions of bytes.

When cameras or cellphones says it takes 10 megapixels photos, it means that each photo has 10 million pixels (mega = million). And having 10 million pixels means 10 million bytes (or about 10 MB) if you want to send this photo (or many photos) to a friend by email, it will have to transfer 30 megabytes of data and it will take a lot of upload and a lot for the recipient to download it later.

One way is compressing the image, which reduces the file without having to resize it. Image quality will suffer as you increase compression and start losing more data.

Another method is to resize your photo, decreasing the pixels it takes to store the image. Doing so doesn't reduce image quality, although it may lose small details.

Photos from modern cellphones and cameras usually have over 6 million pixels, while screens have only about 1.5 million pixels. This means that you end up seeing a resized version of the image (only use the full image if you print it). So if you resize your image, decreasing its width and height to a half, your image would have about the same number of pixels as your screen, and you probably won't notice any quality or detail, even looking at your image in full screen mode.