STEP 1: Begin by selecting a single photo or creating a collage using the layer mask blending method. To learn how to blend different photos you can see our tutorial. Here is a quick arrangement of photos. Notice how the different picture edges aren’t aligned or clean, this is perfectly fine.
STEP 2: Minimize the document, and create a new document 60×60 pixels. Select transparent background. This will be your mosaic pattern. Next, zoom in and take out the Pencil Brush Tool (B) and select a size of 1 or 2 pixels. Draw in a vertical line 2 pixels wide along the left side of the box and a horizontal 2 pixel tall line along the top of the box. You can hold down Shift while doing this to make the drawing line constrain to a linear path. You can use any color (I chose white). 
STEP 3: Go to Edit -> Define Pattern and give it a descriptive name.
You can now close this window without saving.
STEP 4: Now go back to your original document and create a New layer (Ctrl + Shift + N) on top of every other layer like shown. You can call it ‘pattern’. Take out the Paint Bucket Tool (G), select "Fill: Pattern" and then select your pattern from the drop box on the top settings bar. Then click to paint.
Here is what you should have:
STEP 5: Now select the Move Tool (V) and move around the pattern until you have an imaginary white ‘border’ around everything that you will crop. You can see I moved the pattern so that there is a ‘border’ (outlined by the red) which I can crop. This will give a solid white border when you view the finished product.
STEP 6: Zoom in to the upper most left corner of your ‘border’ and begin to crop it where I started. Draw from upper left to bottom right.
Once you crop it, here is what you should have:
STEP 7: Next create a new layer called ‘fill blocks’ on top of everything
Select the Magic Wand Tool (W). Use these settings:
With the pattern layer active use your magic wand to select boxes you would like to be given the ‘mosaic’ effect. Make sure you hold down your Shift key so you can select multiple box selections one after another. To subtract a selection, right click that selection and choose Subtract from selection.
STEP 8: Now with the selections still active, select the ‘fill blocks’ layer and fill it with white using the Paint Bucket Tool (G). This is what you should have:
STEP 9: Next you can lower the ‘fill blocks’ layer opacity to around 50% or so.
Final result 
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