Hi,
It seems your WordPress get different image size version for the_post_thumbnail function. That’s why some post are not aligned.
For example here : http://www.metisteatro.it/category/news/
It displays the post “18.01.13 Lezione aperta” with an image sized 624×826 px and the post “Proposte Spettacoli Estate 2013” with an image sized 100×99 px.
To solve this, you have just to define a rule that says to WordPress to always use a post thumbnail image (of a specific size) that will be generated when you upload an image.
I suggest you to read : How to Create Additional Image Sizes in WordPress, this will help you to create additional image sizes that you will be able to use for post thumbnail. Then all post thumbnail will use the same dimension a,d your posts will be aligned in the 2 columns.
Cheers
mmm i doubt it, because in chrome that don’t happen neither in firefox
also il will happen if i don’t put any thumbnail
just leaving the plain text , title of the post and content and it is not aligned the same