If your website is taking forever to load and you would like to optimize it but don't know where to start, Google PageSpeed Insights Tool is exactly what you need.
Enter your website URL and click Analyze.
You will get a full report on point you need to improve on you website in order to get faster load times.
I will user GIMP and resize all images to a maximum width or height of 1000 pixels keeping the aspect ratio.
Then I use GIMPs' save for web functionality, this way compressing the images.
After updating the images on the website as suggested by the Google PageSpeed Insights tool, we can see a 30% improvement.