Full credit is to be given to fungus amungus for his Instructable mentioned below that inspired this one. Thanks fungus! :)
Okay. Google doesn't offer Picasa for the Mac. Why? Because we don't need it! If you purchased your Mac over the last few years or you've purchased the awesome iLife package ($74US Retail), then you already have iPhoto. iPhoto does everything Picasa does and quite a bit more! (Can Picasa work with RAW files?).
This Instructable is the Mac answer to' fabulous, but Windows-centric Quick and Great Photo Editing with Picasa. To be fair, we will just import a picture, straighten it, color correct it, crop it and resize it in under a minute. While there's a great deal we could do more, this just shows the procedure to duplicate on the Mac what was done on a PC with Picasa.
Note: Like all of my Instructables, this one was done entirely using iPhoto, Preview and Mac OS X's built-in screen capturing features. Okay… Some of my Instructables have diagrams that were created in Appleworks or Eagle.

Even if you can't afford to purchase photo editing software, you can still find free software to create and edit images. Some are developed by individuals, and some are feature limited or an earlier version of a more advanced program. In some rare instances, there are no strings attached, but most often you will need to provide information to the company by registering, or endure ads or nag screens.

10 Easy-to-Use Photo Editing Programs for Newbie Photographers. What you need is a good image editing app 10 Free Photo Editor Tools To Make The Most Of Your Shots 10 Free Photo Editor Tools To Make The Most Of Your Shots. Replacing iPhoto with an image management application that Mac users will actually want to use — but is there. This Instructable is the Mac answer to' fabulous, but Windows-centric Quick and Great Photo Editing with Picasa. To be fair, we will just import a picture, straighten it, color correct it, crop it and resize it in under a minute. Install Movavi Photo Editor on any Apple personal computer compatible with Mac OS versions 10.7–10.13, for example, OS X Lion, OS X Yosemite, OS X El Capitan, macOS Sierra. The app is so easy to learn that you don’t even have to know the basics about how to edit photos on Mac.

Free Mac Photo Editor Download

Though these are all stand-alone applications you also might want to take a look at the free mobile apps from Adobe. They include:

Also don't forget there are also mobile apps from SketchGuru, Skitch, and a number of other Android and iOS imaging apps such as Instagram which gives you the ability to play with images by applying a variety of preset effects and filters to your images.

Finding the Best Photo Editing App for You

The key decision behind using any imaging application lies with what the requirements are for the task at hand. You need to closely research the product and get really clear on both the product's strengths and its weaknesses. Also, take the time to look at the work others have created with the product. For example, if you are looking to create simple graphics or to touch up family photos, then an application without a serious number of filters and effects may just fit the bill. On the other hand, if you want to do compositing and add effects then a limited feature set may not be ideal for your needs.

Also, it is important that you check out whether the application has been updated recently. A lack of updates is the first clue that this software may just be on its last legs. Also just doing a simple Google or Bing search around the application will tell you volumes. For example, Picassa, one of the apps mentioned in this piece has been withdrawn. That's the bad news. The good news is its feature set has been folded into Google Photos which is free.

GIMP for Mac OS X

What We Like

Interface familiar for Photoshop users.
Fully featured, despite being free.

What We Don't Like

Might not be updated frequently.
Lacks adjustment layers.

GIMP is a popular open-source image editor originally developed for Unix/Linux. Often lauded as the 'free Photoshop,' it does have an interface and features similar to Photoshop.

Which pdf editor is the best for mac. I can annotate a PDF. Almost all the PDF editors besides Acrobat can do what PDFpenPro does.

Because it's volunteer-developed beta software, stability and frequency of updates could be an issue; however, many happy users report using GIMP for OS X without significant problems. GIMP is not compatible with Mac OS 9 and earlier.

Pinta

Pinta is a free pixel-based image editor for Mac OS X. One of the most interesting aspects of Pinta is that it is based on the Windows image editor Paint.NET.

Pinta offers the basic drawing tools that you'd expect from an image editor, as well as some more advanced features, such as layers and a range of image adjustment tools. These features mean that Pinta is also a viable tool for users looking for an application to allow them to edit and improve their digital photos.

Image Tricks

Image Tricks is a fun and easy to use free image editor for Mac OS X. It is an application that encourages experimentation and offers the ability for a wide range of effects to be combined and applied to pictures.

Image Tricks is an ideal application for less experienced users to achieve creative results, thanks to the range of filters and masks that are available. There is also a paid Pro version that offers more filters, though you can see the effects that they produce in the free version, without saving them.

GraphicConverter X

What We Like

Works with a huge variety of formats.
Cocooner function allows non-destructive editing, preserving original image.

What We Don't Like

Can be difficult at first.
Cluttered interface.

Free Photo Editor For Mac

GraphicConverter is a multi-purpose graphics tool for converting, viewing, browsing, and editing hundreds of image types on the Macintosh platform. If there is a file format or image processing task that your existing software can't handle, chances are that GraphicConverter can do it if you're willing to tackle the learning curve.

Photo Editor Free Online Photoshop

GraphicConverter is a worthwhile tool to have on hand but needs some serious work in the usability department. The application is not free, but you can use the shareware without time limitation if you don't need batch processing features. Html editor for mac.

Easy Photo Editor Free For Mac