Most smartphones these days take high resolution pictures, it's a good thing, but sometimes the image size can be a problem, resizing a picture on Mac is very easy and doesn't require an extra program, only what comes with the computer.

1. Locate the Finder Icon on your dock and click on it.

2. Browse to the folder that contains the image you want to resize. 

3. After finding the image, Command-Click it, and hover your cursor over "Open With".

4. From the drop down menu, select "Preview".

5. With the Preview window selected, click on "Tools" in the top bar to drop down that menu and select "Adjust Size".

6. On the "Width" field, highlight it and enter the desired width of the image (smaller or bigger than the original one, depends if you want the image big or small).

7. Click "OK", you'll notice the image size changed.

8. Drop down the "File" menu, found in the top bar, and select either "Save" or "Save As". (Save will replace the original image with the resized one. "Save As" will allow you to save the image as a new one under a new name).

9. Quit out of preview by dropping down the "File" menu, and select "Quit Preview".
 
And you're done!