A (better ?) alternative method using Photoshop CS6 exists :
- Import your photos using basics script : file -> Scripts -> Load files into Stack
- Align : Select all the layers and Edit -> Auto-align layers
- Add a Layer Mask to all the layers (all your images)
- Select a brush and paint white/black on the mask to hide/reveal what is behind your current layer
The order of your picture is important ! If the image part you want to keep is on the last image (behind all the others), you have to make the corresponding part of each of the layer mask on top of it black to reveal the good part. Use the visibility icon may help.
I think that this method works better than the mean filter, unlesss you have a looot of noise to remove. Indeed, if you have moving things on your photos, like grass, herbs, trees, a mean filter will yield unnatural effects whereas this method will keep coherent most of the image part.
If you want an illustrated example, take a look here : http://www.photoshopessentials.com/photo-editing/how-to-auto-align-and-composite-images-in-photoshop/