Open the image you would like to apply to Mosaic effect too.
Now we need to apply a smart filter. Click Filter> Then select Convert for Smart Filter.
If you get a message stating that the Layer will be converted into a smart object. Click Ok
Now apply the Mosaic filter. Click Filter> Select Pixalate> Select Mosaic.
Select a cell size, select a size that will still allow you to see the outlines of the image.
I am going to use 34. Make a note of the cell size because you will need it during the next step.
Once you have selected your cell size, click ok.
Now we need to create a pattern.
Click File> New.
Set the Width and height to the same value used for the Cell size. Set the background contents to Transparent.
Now we are going to create a circle inside this image. To get a better view you can zoom in on the image.
Select the Elliptical Marquee tool.
Your mouse pointer should now be a crosshair. Move the crosshair into the upper left corner.
Press and hold the shift key also press and hold the right mouse button, then move the crosshair down to the lower right corner.
Once the crosshair is in the lower right corner release the shift key and the right mouse button. You should now have a selection in the shape of a circle.
Click select. Then click Inverse.
Set the Foreground Color to white. If the Foreground is not set to white press D to reset the Foreground and Background colors then press x to set it to white.
Once the Foreground is set to white press Alt + Backspace to fill the selection with white.
Click Select>Then click deselect.
Click Edit> Then click Define Pattern> Give the Pattern a Name>Then click ok.
Now go back the image, we worked on earlier.
Click Lyer>New Fill Layer>Select Pattern>Then click OK
Select the new Pattern that you just create. It should be the last one in the drop-down list. Then click OK. The Mosaic effect is now complete.