How to see all the image in a header - CSS - image

Question

I want to know how I can see the 100% of a picture in a header with CSS. The problem is that when I add the image to the header, the image does not resize, so I have to make bigger the header, but I don't want that.
.header{
background-image: url("../IMAGES/myimage.png");
width: 100%;
height: 100px;
}
Basically I want to achieve a header with an image resized, so I will be able to see the full image in a header with the dimensions that I want on the header. I am new in CSS. Feel free to ask any question. Thank you.

Answer 1

how I can see the 100% of a picture in a header with CSS
to see a background image in it's entirety, use
background-size: contain;
.header{
background-image: url("//placehold.it/300x100/cf5");
background-size: contain;
background-repeat: no-repeat; /* add this to not repeat it as pattern */
background-position: 50% 50%; /* center it? */
height: 100px;
}
<div class="header"></div>
than id needed you can additionally play with background-position to set it's position
If you want the image to fill the entire header area without distortions use
background-size: cover;
.header{
background-image: url("//placehold.it/300x100/cf5");
background-size: cover;
background-repeat: no-repeat; /* add this to not repeat it as pattern */
background-position: 50% 50%; /* center it? */
height: 100px;
}
<div class="header"></div>
if you don't care that the image distorts but you want the image to stretch-to-fill than use
background-size: 100% 100%;
.header{
background-image: url("//placehold.it/300x100/cf5");
background-size: 100% 100%;
height: 100px;
}
<div class="header"></div>

Answer 2

Without exactly knowing what you really want to achieve, I think you're trying to do, what the background-size property does.
With background-size: cover; you can force the background image to cover the whole area of the according element (like a div).
Your code example would look like
.header{
background-image: url("../IMAGES/myimage.png");
width: 100%;
height: 100px;
background-size: cover;
}

Answer 3

Just use:
max-width: 100%;
On the css of the image, that will make it be at max, the 100% of the width of the container.
CLARIFICATION
For an image inside the container header, max-width will do the trick. If it's a background image, background-size:contain will maintain the image INSIDE the header, while cover will expand the image to cover the whole header.
.header > img {
max-width: 100%;
height: 200px;
width: 600px;
}
.header2 {
background-image: url("https://source.unsplash.com/random/500x500");
background-size: contain;
background-repeat-x: no-repeat;
height: 200px;
width: 600px;
}
.header3 {
background-image: url("https://source.unsplash.com/random/500x500");
background-size: cover;
background-repeat: no-repeat;
height: 200px;
width: 600px;
}
<div class="header">
This is my header
<img src="https://source.unsplash.com/random/500x500" alt="and this is my image inside it" />
</div>
<div class="header2">
This is my second header with a background contained inside
</div>
<div class="header3">
This is my third header with a background covering it
</div>