If they are scanned pictures it sounds like the default dpi is set at 72. If that is the case rescanning at a higher resolution is the best way to do it.
If it is a digital picture it sounds like the person did what I did when I was new. I downloaded the pictures and change the size to 4 x 6 not realizing that I wouldn't get a good print.
If you don't have the original pictures, Miss Tanya gave me a tip. You won't get perfect print pictures but the quality is good enough to print if it an important picture that can't be replaced. For me this was true as they were pictures from my dd's wedding. Here is the tip: Using the percentage increase the picture 110 percent at a time until you get the desired size of picture. I was very pleased with how the pictures looked when I did this.

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Vivian