Preview is a free app that has been included with every version of Mac OS X since, well, forever. It actually originated from the NeXTSTEP Operating System (before OS X existed as we know it today). Most people associate Preview with an app that easily previews images and PDF files (the name "Preview", by the way, is very appropriate). And that's very true, but there is so much more that you can do with Preview.
What if I told you that you could actually use Preview to edit images (similar to what you can do in PhotoShop)? Would you believe me?
In this post, I showed you how to easily add a signature to a PDF document using Preview. There are many other tools built-in to Preview for annotating PDFs. Alongside those tools are tools that you can use to edit images (and believe it or not, they are pretty powerful).
To access these editing tools, we first need to open an image using Preview. By default, JPG and PNG (two common image formats) are already associated with Preview (meaning that when you double-click on the file, the file will open within Preview). If this is still the case for you, simply double-click the image to open it in Preview. If these file types are associated with another application (this can happen without your knowledge when you install other image editing apps), we can still open the files in Preview. To do so, right-click (Ctrl+click) on the file, then choose to open the file with Preview.
Once the file is open, we need to open the image editing tools. To do this, go to Tools > Adjust Color... This will open a new HUD (Heads-Up Display) with lots of editing options:
As you can see, using this HUD will allow us to control many aspects of the image, including exposure, contrast, highlights, shadows, saturation and many more. It even gives you control over the image's levels, which allows you to control each color (red, blue and green) independently of each other. Here's a sample edit of the image above using these tools (applying only subtle adjustments):
Once you've made your edits, go to File > Save to save the changes.
Note: once you have made changes, saved and closed the file, there's no way to reverse the edits that you have made, so I would advise making a copy of the file before you edit it (just in case).