Whatever the source - scanner, created, made with plasticine and photographed etc, if you have a digital image you can rescale it. Most standard graphics software packages - GIMP, Paintshop Pro and so forth - will have a facility to do this. With something like GIMP you load up the piccy, chose Image-Scale and play with settings and look at the results.
Then save the image as some other filename or type. Actually, to be safe, get into a habit of "Saving As" some other filename before you start tinkering with your original.
When saving compressed image formats like jpg, look out for the compression level options in the save dialogue. The choices are all about compromise.
It also likely that your scanner software, or a freebie package that came with it, can rescale images.