It's simple, using Photoshop, select the layer where the image is located and add a layer mask in the bottom of the layers pallete. Now using the brush tool, select a brush of you choice, preferable a grudge one, also select the size of your brush, use it only in different areas of the image, DONT use it everywhere or the smudge effect will get lost.
You could use it directly over the image without the layer mask, but I am not a image distroyer so I like to preserve the originla in case you need to change something back to normal. I'll open up my photoshop and try to take a few snapshots to show you, but you can try with this for now.
Good luck.
~J.