PhotoPad is a desktop photo editing and filtering tool that operates like and contains many of the same features as a mobile editor. For those that share a lot of photos on Facebook, Twitter, and Instagram, this is actually a very good thing, allowing you to download your images from a phone or tablet and edit them on a much bigger screen without needing a resource-hogging, expensive photo editor.
PhotoPad installs to your Application folder, but because it is a third-party app you'll need to set an exception for it before loading. Once done, it opens without issue and allows you to get to work immediately. You can edit photos, create collages, or run through a quick start tutorial to learn about all of the features. There are quite a few features, so those new to the game might want to start with the tutorials. While no one will confuse this for Photoshop or even some of the lower-cost photo editors on the App Store, it is a very useful tool that offers many features iPhoto does not -- things like filters, contrast and coloring adjustments, and more. The interface feels dated and pixelated, and while it can access photos anywhere on your device, iPhoto integration is limited, but for straight editing of photo files, it works wonders. There was an occasional lag when working with larger files or moving between editing menus, but these were minor issues, overall.
If you want a photo editing app for your Mac that won't run slowly, overload you with features, or cost an arm and a leg, PhotoPad is a good option to start with. It's not a super-polished app and it has some performance issues, but it works well most of the time and is a good app because of it.