A quick list of minimum image sizes
You can have images larger than the sizes listed, however they might be cropped (from the center) to fit the aspect ratio.
Images that are smaller will be stretched or magnified which could result
(width x height)
Main Slider - 2000px x 1347px
Alternate Slider - 2000 x 600px
Testimonial Featured Image - 160px x 160px
Client Logo - 200px x 58px
Meet The Team Featured - 640px x 330px
Meet The Team - 320px x 330px
Portfolio Featured Image - 960px x 620px
n.b. project page slider height depends on portfolio image height.