Dear Roi, I was facing same problem of “wrongly cut” thumbnails (it was cutting images in the middle). I found this post and following Ronnel instructions to set the size in WP-Admin -> Settings -> Media and unchecked following option right below the 150 x 150 thumbnail size
Crop thumbnail to exact dimensions (normally thumbnails are proportional)
Then I deleted manually previously generated thumbnails from media library, and re-generated thumbnails… My problem resolved and now all the thumbnails are rightly cut 🙂