How to stack dual images in one ?

Simple enough it seems, but I can't figure it out... how do you stack two images in one like this example ?:

http://www.flickr.com …8/sizes/l/in/ph​otostream/

What software?
Photoshop:
Open two the images.
In one image go Image > Canvas Size.
Double the canvas size, tick the top box, so the canvas expands down only:

IMAGE LINK: https://skitch.com/ren​edamkot/r68ce/canvas-size

awsome, thanks.

FYI, if you don't want to bother with having to open two or more files into separate workspaces, and then having to drag-n-drop them all into one workspace, you can use the File > Automate > Photomerge command.

Browse to the files and select them, and then de-select the "Blend Images Together" option. This will import the files into their own individual layers in one single workspace. If you are using CS5 and want the the Interactive User Interface that allows you to place the file that was available with previous versions of Photoshop, you can download the additional plugins here:

http://www.adobe.com …ads/detail.jsp?​ftpID=4688

Probably not a huge time saver if you're just doing 2 files, but would save time using the Photomerge command without the blending option if you have several files you want to place or manipulate as separate layers.

File > Scripts > Load files into stack or
Bridge > Select images > Tools > Photoshop > Load files into PS layers...
would also work.

Use Comic Life and you can have all sorts of crazy layouts...