Header [theme Subtle] some pics work other don't

Good morning,

I have a question about a header in theme Subtle. I hope that someone is willing to help me. I created a new header. This pic larger than required (1280 pixels wide and 416 height, resolution 300dpi), but I can use it without resizing. When I look at the site the header starts at the left side of the screen up until the end of the screen. Exactly how I want it. Now I have to replace the image by another one. I used exact the same size as the other one, but now WordPress tells me that I need to rezise and create a small banner. That is not what I want.

How can I use this picture as a header without resizing, like I did with the first one.

Thanks for helping me!