Why are people recommending such expensive software to the OP? Adobe Photoshop? Really? He didn't say he needed extensive photo-editing software. And somebody recommended iPhoto which already comes on the Mac.
@OP I would highly recommend that you do take a good look at the Mac App Store. There are a lot of good alternatives to expensive softwares already recommended here. For example, you can try "PixelMator". I use it constantly for our office to touch up and enhance photos for banners, business cards. Has very good layering features and blur options. Unless your office requires Adobe, you won't look back at Photoshop once you use Pixelmator. It's only $15 on the Mac App Store.
https://itunes.apple.com/us/app/pixelmator/id407963104?mt=12
Also, the Mac App store does show the top 10 downloaded softwares.