Images in box, it's not about pixels I guess, but too big compression of used photos.
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Please check:
K2 image settings in options
GK NSP Thumbnails Quality settings
next
upload those 6 images again,
there must be at least this resolution 1 045px × 653px !!! High Quality JPG, low compression.
Why? because first K2 will compress them , then Gk module .... so finally you get what you seen now, LQ