Creating a Mosaic in Photoshop
I have a great tutorial for you here on turning an image into a mosaic using Photoshop.
There are a lot of different ways to do this, each with a different result. I’m not saying mine is the correct one, or even the best. It just has a really cool screen printed look when finished, which I really like.
Start of with the picture of your choice, then follow these steps.
I choose this image of a woman from iStockphoto because of the retro coloured background.

Step 1 - Go to your Filters menu and hit Pixelate>Mosaic. It is up to you how big your size of mosaic is. I have used a larger image, so mine is 28. Just use your best judgement.

Step 2 - Next, you are going to open up a new photoshop file to make a pattern. You will need to make it the same size as your mosaic (mine will be 28 x 28). Put a black background on there, then use the Marquee tool, to create a proportionate circle two pixels smaller (26 x 26 for mine). Fill the selection with white. Go to Edit>Define Pattern.

Step 3 - Now go back to your mosaic file. Bring up your blending options for your mosaic layer and apply the newly created pattern as I have here. Make sure you change your Blend Mode to Soft Light. If you want, you can change the opacity to soften it up a bit. I have left mine like this, because I like to contrast.

Step 4 - Here is our finished result. You can play around with different images and blend modes to achieve exactly what you are looking for. Or you could always invert the colours to get something really crazy. Its up to you. Have fun.

