Before you buy any of these apps, you should give them a go by using the trial versions. It's an easy way of seeing if the app is right for what you need to get done. This way, you'll ensure getting the ones suited best for you.
2. SketchBook Pro As with many apps, you can try this one out, but it will cost you $50 at the App Store. It's mainly a way to create images, like what you'd use Adobe Illustrator for. There is a free version of this software in the Mac App Store and it's called Sketchbook Express.
4. GIMP For Mac This is an open-source freeware that will allow you to edit your images. There are many ways of using it, from a simple paint program to an expert quality photo retouching program. Since it's free, it's hard find reasons why you shouldn't give it a try.
5. Adobe Lightroom This is available for Windows and OS X and it's a strong tool to do image post-processing with. There are a lot of features that will allow you to play with your images easily, as well as a lot of set options, which means that you can use them on your photos without too steep a learning curve.