Here's a walkthrough of Pixelate Photo Privacy:
The utility opens with a sample image to practise on, which is a nice touch. You can splay and pinch to zoom in and out, as usual, and pan around by dragging the image. There are tools for blurring out, for blocking completely and for applying a linear smudge. Above, right, I've applied the latter effect by dragging out a rectangle - the tool shows the area that will be rendered with a hatch pattern - it's not a real time effect.
After tapping on 'Save' the effects are rendered. I've then opened the new image and zoomed in again to show the pixellation I just added. You can vary the severity of the pixellation in Settings, though it's a trial-and-error numbers thing only - there's no graphical indication of what each number corresponds to on-screen. By default large (e.g. 12MP) images are scaled down to save on rendering time, but can uncheck the top box (as I've done) to save at full resolution. Rendering times are only a few seconds, so it's not a big issue.
Here's a real world example of myself and my dad. Choose the pixellation tool needed and then drag out a rectangle over the face (or sign or confidential detail etc.) Then tap on 'Save' and you're done. I've used Pixelate Photo Privacy itself to re-open the new image, so that you can see the pixellated effect.
Rather cool and rather useful. There are various social sharing options too, to Flickr, Picasa, and Twitter. If you're a Harald Meyer fan like I am, then head over and grab this from the Nokia Store, currently at £1.50.