I've made this response to somedy else :
In fact when you insert image into your wordpress with this theme, the theme create automatically thumbnails of your images with several size (110x70, 150x150, 300x200, 470x140, 470x260, 750x380 px), when you put image in feature, the theme use theses thumbnails (470x140 and 470x260px, i think).
So, in your ftp, go to wp-content/upload/.... and substitute the thumbnails with the "good" size image (you had to resize with photoshop before at 470px of width and what you want for heigh). The file's name must be the same than the thumbnail.
It's not a real solution but it work fine...
PS : I apologize for my english, it's a disaster... (I'm french)