This type of graphic can look very effective on websites or brochure covers. It gives the illusion of a cut-out through which underlying content can be seen.
Its very simple to create in Pixelmator. Here is a quick step by step that shows you how.
Start with a new document. The one above was sized 800 x 600.
Colour the background layer using the paint-bucket tool. In the example above the colour was #a36053
On a new layer create the shape you would like to use as the cut-out. The example was a double arrow, but it can be any shape you like - or even text. Fill the shape with Black and leave the stroke as none.
Next, select your chosen image for the background and add it as a layer ABOVE the shape layer.
Right-click the image layer in the layers palette and select "Create Clipping Mask". Your image will now be clipped to the shape added to the shape layer.
Select the shape layer and add an inner shadow using the styles palette. Choose black as the shadow colour and set the angle at about 130 degrees. set the shadow offset at about 12 pixels and the blur at about 25 pixels - or simply adjust to your liking.
Add some text and export your image.
Here is a screen-shot of the final layers palette.
As an alternative to using an image, you could just use a filled layer. The example below uses a plain white layer as the image layer and text as the shape layer.