Featured Image won't work

Hey everybody,

I have strange problems using this function of WP.
I don’t have troubles activating it, it is working and I can use it.
I first upload a picture to the post and put it. That’s ok.
I have set up a default thumbnail size of 260×60, which are automatically created. Every time I upload a photo, the thumbnail size appears correctly, in fact I can add it to the post and the size is correct.

The weird thing is that when I click on “Set feature Image”, pick the “thumbnail size” and click on “add as feature image”, it won’t add the thumbnail size but the full size.
The only solution is to manually crop it with the WP editor, BUT this is also weird because the default image to edit is a strange size, it’s never the one that I selected, so when I resize and then crop, what I though I had selected was something totally different, and sometimes even a black space (which means I was cropping “outside” the picture).

Any help would be really appreciated, thanks!