If only parts of a photograph are blurry (at similar distances from the camera) it's a focussing problem. If the whole image is blurry, then it's due to camera shake.
Either way, it's impossible to add detail to the photo which wasn't recorded by the camera in the first place. You can get a slight increase in the overall 'sharpness' (but with an increase in 'grain') by using a program like Adobe Photoshop or the nearest free equivalent to it, Gimp:
http://www.gimp.org/
It's pointless paying any commercial service for such a task as:
(a) the high cost wouldn't be justified by the fairly poor results ; and
(b) you can achieve the limited improvement that's possible yourself anyway.
Chris