But first a small Photoshop tutorial, so - here we go (all credit in the world goes to Mike at Santa Cruz Warhammer for his October 2009 article that taught me how to do this):
Open up your image in photoshop. Once it is open add another blank layer to it:
Go to 'Layer'
Then to 'New' and select 'Layer...' then click 'ok'.
Make sure this new layer pops up above the layer with the pictures of your minis. Now make sure this layer is selected so that this next step is done in the new layer, and not on your minis!
Go to 'Filter'
Then go to 'Render'
Then select 'clouds'
You will now have a picture full of black and white clouds. Select your eraser and set your opacity to about 15% with a pixel size of about 20. Begin at the center and erase in circles and you can see your minis start to appear again. Take these circles all the way towards, but not over, the edges. Now increase your opacity to about 20-25% and do the same thing but now stay closer to the minis. Finally put the opacity to 40% or so and do the areas just around the minis.