Irfanview, only available for Windows Operating System, is another free choice for enlarging pictures with little loss of quality.

It is available for download here:

http://www.irfanview.com/main_download_engl.htm

Open your image in Irfanview and go to Image » Resize/Resample.

You will see the image re-size popup window. Enter your image size in the Set Image Size. Now you need to select re-sample under the Size method. Use the filter drop down menu to choose Lanczos (slowest). As a last step, check the box next to ‘Apply Sharpen After Re-sample’.