thumbnails and thesis_post_image not being displayed anymore?

Hi,

Ever since I upgraded to the latest version of WordPress (or more precisely ever since WP 3.0), my thumbnails/post images (before/next to post headline) don’t appear at all. I am using Thesis 1.8 as my main theme.

I thought at first the problem came from the theme itself, and ‘thesis_post_image’, but since everything worked fine until I updated WP, it seems doubtful.
I even checked via FireBug if the thesis_post_image was somehow hidden (or on display:none) but there is no trace of any thumbnails, as if the custom field isn’t even executed.

Maybe WordPress’ new thumbnail options render the Thesis post image thing nonoperational?
Any other idea on what causes the problem and ways to fix it?

Thank you in advance for your answer.