picture header is different if it is a phone, pc or eeepc

Hello,
I use the free theme catch Box on http://www.randomarmotte.com.
I put a picture in the header.
First, I used a child theme.
1- I try to put a picture with the recommanded size in the option (320 pixels and 120 pixels) but the picture was very little.
2- I put a picture which the size is higher, then I reduce the marge of the header. For me, it was OK but I discover on my mobile phone or my eeepc, the picture of my header go out the header. I didn’t understand why ?
Can you explain why it is not responsive ?

Thanks.