I'll never forget the first time I saw a real photo mosaic. It was a photo of Marilyn Monroe on the cover of a magazine consisting of hundreds of images. It was one of the coolest things I’d seen at the time and wondered what kind of amazing technology was used to create such an amazing piece of work.
I can tell you there is really only one place to get a true mosaic like the one I saw. This is at Picture Mosaics, but you will also pay a couple hundred bucks for it. There is a poor man’s method for producing a photo mosaic. It's nowhere near the wow factor that a true mosaic is, but it's still pretty darn cool and easy to do. Some companies out there will charge you for this, I'll show you how to do it for free.
Start by copying photos into a new folder called Mosaic. I just copied and pasted images at random and ended up with 550 of them. Be sure the images in this new folder are copies and not originals.
Next, you want to start an action. To do this, open up the first image in your new Mosaic folder and open the Actions palette. (Windows>Actions) Now click the arrow in the upper right hand area of the palette and select new action. Go ahead and name the action, I named mine re-size.
Now go to Image>Image Size and check Resample Image, make the image size 1cm X 1cm at 200ppi and click OK, save the image and close it. Now go back to the Actions Palette and click stop. You've just recorded the image size adjustment.
The next step will be to turn your action in to a droplet. Go to File>Automate>Create Droplet. Click the Choose button and decide where you want to save your droplet, select the re-size action you've just created and click OK.
Now go back to the new folder of copied images you created, select them all and drag them over the Droplet icon. Make sure you are dragging the copies! This will convert each image to a 1cm X 1cm square.
Now that all the images have been reduced to their smaller size, it's time to make the mosaic. To do this click File>Automate>Contact Sheet II.
Select the images you want to you use by clicking Choose and then enter the measurements you want to use. I used 55cm X 40cm and 200ppi. Now decide how many rows and columns you'll need. I chose 50 columns and 30 rows and made sure to uncheck Auto Spacing so I could make sure there is no spacing between the images. Since the images are square they will fit together nicely. Check the Flatten All Layers box and click OK.
Here are the small images put together in the contact sheet. If you didn't have enough images, simply copy and paste them to fill the canvas, just be sure to Flatten the image after you do by clicking Layer>Flatten Image.
Now decide what you'd like to use as the main image, but be sure it's at least the size of your canvas. Now click File>Place and find this image. This will place the image in a new layer above the smaller ones. Set this image layer's options to Hard Light.
That's it! You can play around with the Curves on the bottom layer if you want to adjust the contrast a little. That's up to you.
There are so many ways to do things in Photoshop. This method is one of the many I may use for a given project. You may have a better way. Always do what works best for you.