There are many ways to blur people's faces so that they are not recognisable. The easiest way to do this, perhaps, is using Gimp's "Smudge" tool. Gimp is an open source image manipulation programme and can be downloaded here
Open your image with Gimp.
Now the image is loaded and appears in a seperate window.
Choose the "Smudge" tool in the tool panel.
Apply the smudging by point to right places and pressing the left mouse click until the face is blured and unrecognizable.
exif thumbnails do not get updated automatically when changing the image in Gimp. There is, however, an exif thumbnail editor contained in Gimp 2.2-pre2 and greater.