d3v1d14 on "Picture in post gets contrast change when shown in blog preview"

Hi,

I use the function so that tha picture i use in a blogpost, is automatically shown on my homepage as preview. Now sometimes this picture is shown black(ish) while the picture is white, or vice versa. How come and how can i fix this?

example: https://sportsrecapcenter.wordpress.com/2018/04/15/ronde-van-vlaanderen-2018/
https://sportsrecapcenter.wordpress.com/

thanx!