You can set iPhoto and Aperture to use an external app to edit photos instead of merely using the adjustment and filter editing in the apps. You can set iPhoto to use Pixelmator, but Photoshop works more smoothly when trying to save back into your library.
The latest version of Pixelmator introduces a handy new way to select part of an image. You can paint with a brush to add to or remove from a selection and cut out a person or object. You can then apply filters to that selection, or invert the selection to apply filters everywhere else.
You can use a graphics program like Pixelmator to add some semi-transparent text to your photos before uploading them. Adjusting the color and opacity of the text will make sure the text doesn't interfere with the photo's content, unless you want it to.
You can use Pixelmator to add text directly to an image. You can use different fonts, sizes and styles. You can also rotate the text and add a drop shadow. Pixelmator special effects can be applied to text once the text has been converted to a bitmap layer.
There are a variety of effects in Pixelmator that you can use to make subtle or extreme changes to your images. You can apply simply adjustments, or add new elements. You can apply effects to an entire image, or just a selection. Take a look at some examples and then try to play with effects yourself.
Learn how to use the clone tool in Pixelmator. This drawing tool can be useful for adding additional copies of repeating objects, like flowers in a flowerbed, to your photos. You can also use it to clone an uneven background to remove objects from a photo without leaving an obvious empty space in the photo.
Learn how to use layers in Pixelmator. You can place one image on top of the other and then use masks to blend them. You can use a simple clipping mask to quickly reveal only part of an image. You can use a layer mask to create cool effects like blending two images together gradually.
Learn how to crop photos using Pixelmator and then resize and export them. You can also cut out a part of an image and trim it. Pixelmator has a variety of ways to modify a selection to create better cut outs.
Learn how to create transparent images for use in Pages, iMovie and other applications. You can cut out part of an image using the app Pixelmator and that part of the image becomes transparent and shows the items behind it.