".......If you have a computer, they usually come with some basic drawing tools like "Paint" or use paint.net.... Take the image and open with Paint, then just use the rectangular select tool to pick what you want to enlarge. Then just crop it and you have a nice separate image to save........"
The fully-featured Open Source image processing package is called 'The Gimp'.
You can download it for free, and it enables you to perform any image manipulation process you like - matching professional packages like Photoshop.
I use it to create full drawings of vintage model boat plans which are often sent to me as a set of partial A4-sized scans. These can easily be re-sized, rotated, matched up and stitched together to make a full-sized drawing. For example, look at any of the Stirling plans on my Old Boats website:
http://oldboats.tk/Sterling1.html