Sounds to me like the OP has a small size picture that they wish to enlarge to be their entire background, but when they do this its very "blocky'.
And the answer is essentially "thats what happens", you cannot put back details that aren't there.
Unlike in CSI where they can enlarge a single pixel to show someone's face or a number plate, in real life a single pixel enlarges to a bigger single pixel. There are some picture editing apps that can make an attempt at smoothing out and guessing where detail is by looking at neighbouring pixels, so the result looks less blocky, but even the best of these will have severe limitations on what can be done. Without knowing the size of the original picture in pixels its not possible to know if one of these apps has a chance of working any CSI magic.