I have lost the original of my photo, which was about 500x500 pixels, JPG. The only one survived copy is 200x200 pixels. So, I want to enlarge it back to original size, and obviously, the main problem is noticeably reduced image quality, i.e. after enlarging 200x200 image back to 500x500 it becomes very blurred.
What tutorials or recommendations I should use to restore (as much as possible) it's original quality, i.e. make it sharp?