I wouldn't reccomend reducing the quality, what you need to understand is 'resolution'. Reducing the quality just makes the image look crap, it won't actually shrink it suitable for screen.
Your monitor is somwhere around 1024x768 to 1600x1050 pixels, where as 3008 x 2000 or more is a typical resolution straight off the camera.
What you need to do is use a graphics program ( I use Paintshop Pro on my PC and Photoshop on my Mac, it's doable in elements too but not so flexibly as the other two ) to reduce the resolution.
For example in PSP I go to the image menu, select resize. I then make sure the aspect ratio is maintained ( often a chain between the Height and Width indicates it ) and change the width of the image usually to 800 pixels.
When that's finished then I usually save as a png not a jpg as the quality is better.