It's not so much a resizing issue (consider the corners of the photograph, where bends / folds are relatively clear), instead it looks to me like this image was scanned at too low a colour depth (probably 8-bit) and later upscaled to 24-bit.
What this means is that the digital image doesn't have enough colours in its palette to accurately represent the many shades of grey in the photograph, and they get averaged out - resulting in large blocky areas of one colour.
If you have the original photograph (and it doesn't really look like this), try increasing the colour depth when scanning. If not, unfortunately this isn't something that can be fixed with digital editing. You can turn a block of multiple colours into one colour, but you can't go the other way.