Image "à la une"

Hello !

I just wrote a new post on my website (http://www.judo-vsg-fribourg.ch/), with an image. As I publish my post, I have to open the full article to see the image. Unless I put an image “à la une” (french translation). Then, I can read my article twice : the normal post and the “à la une” version. But my image is cropped! Which is the correct size of an image to be put “à la une”?

Thanks a lot for the tip!