The Chromebook photo editor is not a Chrome app, as you might think. It's integrated into the file manager. To use it, launch the file manager by hitting Ctrl+M, then navigate to where your photo is located.
Once you click on the photo you want to edit, two options will appear in the lower right-hand corner--View and Edit and Delete. Click on View and Edit to launch the photo editor.
You can allow Chrome OS to autofix the image or crop it.
You can also change the brightness and contrast, or change the orientation of the image.
That's all there is to the Chrome OS photo editor. It's about as basic as they come, but if all you need to do is make a few basic edits, it's definitely worth having, especially if you don't have an Internet connection. If you do have an Internet connection, you may also want to consider using other, more powerful Web-based photo editors.