Some Images Losing quality

Hi, on the link to my website there are 6 images, the 4 images closest to the top have clearly lost quality and are not at the standard I need, however the bottom 2 images are the ideal quality. I have checked the image data and they are all approximately the same size (0.9 – 1.8 MB), exported at lower/higher qualities yet with the same result. Any help would be greatly appreciated.

The page I need help with: [log in to see the link]