Display larger 1749 * 626 Image

When inserting media, did you check “full size” in the ATTACHMENT DISPLAY SETTINGS?
You can add the image (in the text tab, not visual) by something like this :
<img src="http://your-site/wp-content/uploads/your_img_1749_629.jpg" width="1749" height="629" />

Do you want this to be a fixed background image?
If so you add it in CSS (customize this to your liking):

body.yourcustomstyle {
     width:100%;
     overflow-x:hidden;
     background-color:#000;
     background-image:url('/wp-content/uploads/path/filename.ext');
     background-size:cover;
     background-repeat:no-repeat;
     background-attachment:fixed;
     background-position:center top;
}

To discover what CSS is output by your theme, use a web inspection tool such as Firebug: http://getfirebug.com/ , Chrome Inspect Element: https://developers.google.com/chrome-developer-tools/ or Internet Explorer 9+ F12 Dev. Tools: http://msdn.microsoft.com/en-us/library/dd565627(v=vs.85).aspx#csstool

There are others.

When editing CSS, use a Child Theme
http://codex.wordpress.org/Child_Themes
Custom CSS Plugin, or Theme provide custom CSS option.
Edits to parent themes are lost on theme update.

Learn CSS: http://www.w3schools.com/css/