Preparing images in Apple Preview

Apple Preview is a simple program that is pre-installed on most Apple computers. 

If you don't know where Preview is on your computer, go to the Finder > Go > Applications and look for Preview.

Look in the Applications folder, and click Preview to launch it.  It should prompt you to select an image.

After selecting an image, go to Tools > Adjust Size...

Make sure the Width and Height settings are in pixels.  And make sure the lock icons next to it appears locked.

If the Width or Height is longer than 2500 pixels, change it to 2500 pixels, and click  OK.

Then, go to File > Export...

Make sure the format is set to JPEG with the quality at around 60 - 70%.

You may want to give your image a slightly different name, just to help you keep track of it.  And then click Save