post 
id, title
1   Заголовок

post_item
id, post, sort, image
1   1       2    img1.jpg
2   1       1    img2.jpg
3   1       0    img3.jpg

post, title,           image,     image2
1     Заголовок       img3.jpg   img2.jpg

select p.*, i1.image, i2.image image2
from post p
left join post_item i1 on i1.post=p.id and i1.sort=0
left join post_item i2 on i2.post=p.id and i2.sort=1

select
  p.*, 
  max(if(i.sort=0,i.image,null))image,
  max(if(i.sort=1,i.image,null))image2
from post p
left join post_item i on i.post=p.id
group by p.id