I run into this problem often. People take photos of things with their cell phones or a small pocket camera set at a low resolution. While photos like this are fine for viewing on a computer screen, they are not always suitable to print or print at an enlarged size. One project, the file size/resolution was so small, that full size printing of each photo was only about 1/2 inch tall by 1 inch wide. The photos my recent client brought me could only be printed at about a maximum of 1 1/2 inches wide by 2 inches high. He had hoped that at least one of the photos be printed much larger. Low resolution photos that are printed larger, end up looking blurry.
So remember, size matters. When taking photos for printing or enlarging, it is best to use a high resolution setting on your camera. Photo images can always be made smaller when they are too large, but it is impossible to make images larger when they are too small.