Increase the size of IMAGES without noticeable loss of QUALITY
Images, whether a .jpeg, .png are most commonly displayed on the web as a bitmap. A bitmap is a piece of text, a drawing, etc., represented, as on a computer display, by the activation of certain dots in a rectangular matrix of dots. When all these small squares are compressed, they become what appears to be text or images. How well compressed these squares are determines the quality or what is represented. The more squares in an area, the better the appearance to the human eye, up to a point, of course.
When you look at a photo and zoom in, you will be able to see what really "makes up" the picture.
The usual way to decrease or increase the size of an image is to decrease or increase the size of the small square, or pixel. Making things smaller increases quality, but making things larger tends to degrade the image and make things look grainy, blurry or, in tech terms, pixelated.