We start with the original image devoid of penguins ( unless they've perfected stealth technology since we last checked ). The first thing we're going to do is add some of them to the ice floes in the foreground. Obviously we want the penguins further away to look smaller and we don't want them to look too bright as they'd stand out from the image too much if they were.

Start by adding a new layer over the top of the base image and filling it entirely in white ( I could make jokes about polar bears in snowstorms here but I won't :) ). We need to paint in areas that we want the penguins to appear on and this is going to be somewhat difficult unless we can see the underlying image, so lower the opacity of the top later to around 50%. Now take a brush tool and paint in some black lines where you want the penguins to be standing on top of the ice floes, like the example below.

Particle Panel

The penguin particles are loaded from left to right across the top row, penguin1 to penguin5. The column select should be set to 'Random' as we want a good spread of the different penguins without any contrived looking patterns if we can avoid it.

Packing Panel

Set the jitter amount fairly high ( I used 95% but it's not terribly important to have a precise number ) and choose 'Luminance' as the Density setting. At 0% set the slider to 58%, and at 255 set the slider to 0%. This means that wherever the layer beneath is black the penguins will appear but where you have filled it with white you should see no penguins at all. At this point you'll see the pattern of penguins that you were looking for in the preview pane. Check 'Clear Underlying' in order to make sure that when you apply the white and black layer is removed and only the penguins appear on your final image.

Color Panel

We don't want all of our penguins to look the same and it's going to be a good idea to make sure that we match the brightness of the penguins with the area of the image we are placing them in if possible. Set the Hue to 'random' with the minimum value set to 49% and the maximum value at 51% and do the same for the Saturation setting. This gives us a small amount of variation between individual penguins. As the foreground of the image where we are placing the penguins is not terribly bright let's set the Luminance slider to 'random' with the minimum value at 32% and the maximum value at 34%.

Variants

Here's where we start to get a feeling of depth by making the penguins further away smaller. Set the Scale slider to 'Vertical Distance', 10% at top, 38% at bottom. Setting the Rotation slider to 'random', minimum value 0%, maximum value 5% makes sure that all of our penguins aren't standing at exactly the same angle.

Shadow

The light looks like it's coming from the right and above. If we have a heavy obvious shadow to the left of the penguins then they will be casting it on the air or an ice floe behind them by a few metres which could look a bit odd. I set the shadow sliders as follows. radius 42%, Opacity 45%, X Offset -2%, Y Offset 9%. This makes the penguins feel less "stuck on" as they now have a bit of a shadow under their feet. Now apply Scatter! If the penguins are a little too low or too high on your image, just move the layer fractionally up or down until they look about right.