printing photos in poster size

Hi all, has anyone had any luck printing posters/pictures, for their walls, from a photograph? Ive tried doing it online but my photo quality isnt strong enough. Someone suggested that there are walk-in printing shops where they can fill-in/grade the pixels in a poorer quality photo.

Any help would be great

Photobox http://www.photobox.ie will grade each print before you order it to see if it has the correct resolution. Also they have a technical guide of recommended resolutions for each print size.

Harvey Normans in Naas do it -no affiliation, I have just seen some of the pictures that were done, they also put them on canvas if you'd prefer