Header size problem

I use Chiron theme, so I went to the editor and tried to change the header size in the code.

.container_header_top {
	padding-top: 0px;
	padding-bottom: 0px;
}

#head-search {
	margin-top: 0px;
	float: right;
}

#header {
	width: 100%;
}

#headimg #logo-image {
	width: 800px;
	overflow: hidden;
}

#headimg #logo-image img {
	width: 300px;
	height: 75px;

	border: none;
}

#headimg #logo-text {
	margin: 12px 0;
}

#headimg #logo-text .site-name  {
	display: block;
	font-family: 'Oswald', sans-serif;
	font-size: 28px;
	line-height: 34px;
}

#headimg #logo-text .site-description {
	display: block;
}

I tried to change the
#headimg #logo-image img {
width: 300px;
height: 75px;

and

#headimg #logo-image {
width: 800px;
overflow: hidden;

to width: 1000px height: 274px. I then found a picture that was 1000px – 274px.
But when I look at my website the header is in low quality (it’s high quality otherwise) WEBSITE HERE

Can anyone help?