Screenshots are handy when you have to explain things or elucidate how-to articles through images. People can see the visuals cues (images) and understand them faster. However, the addition of arrows makes things simpler. Adding arrows to your screenshots is no rocket-science. They come in your Mac’s Preview app. You just have to know how to use it.
With arrows, you can highlight the major points/areas of your screenshots, which otherwise would be confusing for some.
Here is How to Add Arrows to a Screenshot On Mac OS X
Step 1: Open your screenshot. Normally, it opens in the preview app. In case, if it does not, then click on Applications → Preview App → and then open the screenshot file within the app.
Step 2: Click on the toolbox icon on the top-right.
Step 3: Locate the “pencil” icon beneath the menu bar of the preview app window.
Step 4: Click on it and you will see the annotation menu with various options.
Step 5: Select the “Arrow” option from it.
Step 6: A new arrow will be placed over the image when you click on the arrow icon. You can drag the points on it to change the size, direction and location.
Step 7: Other tool options can be found in the toolbox. You can thicken the arrow line, change its color and more.
Step 8: Click on “File” → “Save a Version.”
Note: While you are done drawing the arrow/pointer, you can also change its color, thickness, and length. If you want to shift the arrow, you need to click and drag it.
Also note that it's important to save the file in Preview (as long as you're not running Yosemite). In Yosemite, the file gets auto-saved (or that's how it works for us when we close the Preview app without saving the file). But just to be sure, you might want to save the file in a new version to retain the original one too.