Terrible quality photos

Hi,

I really hope someone can help….

My photos on some posts have a terrible quality. Even though they were uploaded a decent size. Once i go into edit post, they look nice and good quality. It only goes horrible once posted.

I am pretty sure it’s something to do with resizing, but I can’t work it out how to fix it. Please help, I spent too much time on this now. ??

Example -> https://www.mylifemypassion.co.uk/2015/03/26-weeks-pregnant.html
Thank you