Is the picture your trying to enlarge a photograph or a printed picture? A photograph will have continual lines and have all shades of gray. A printed picture will be a printed in black and have a screen so the picture will be made up of tiny dots with white space around then to give the appearance of gray, on this type of photo when it is enlarged all your doing is enlarging the dots and the white space around them. If it is a photo use your flat bed scanner and scan the photo at the highest dots per inch and maximum size your scanner can deliver. Next resize the photo to your target size, keep in mind that most digital cameras only save photos at a resolution of 72 PDI. Other things that can effect the output is the paper, if it has a texture the scanner will pick it up and it will get enlarged in the process and show in the final enlargement. You can enhance the image by applying filters and effects. De-speckle, adjust contrast and brightness, sharpen edges, use sharpen or soften on the entire photo or parts of it. Another thing to keep in mind is some programs have a limit on either the dimension of, or the size of the file. If the photo you are working from is good and clear and on a flat glossy paper you can get great results, I have blown up pictures as small as 2" x 3" TO 2' x 3' and they've turned out great. good luck.
