Image slider – photos are blurry but sized correctly and 4k!

I uploaded 5 images on my slider. All the photos were taken as either 4k photos or high res photos. However, when I size them according to your size parameters, they become blurry. I did compress them in the tinypng app because they were obviously very large in size (more than 1 MB each), but that should make the quality so poor. The photos in your demo are crystal clear. What am I doing wrong? Thanks!