Expand an image in a page

Hello forumers ! I have a doubt.

Well, what I’m planning to do is to insert an image in the page that takes all the width of the page and that image shouldn’t have any border.
What I have is this:
[URL=https://imageshack.us/photo/my-images/707/31403716.png/][IMG]https://img707.imageshack.us/img707/6853/31403716.png[/IMG][/URL]

And what I wanna do is this:
[URL=https://imageshack.us/photo/my-images/845/85708024.png/][IMG]https://img845.imageshack.us/img845/8453/85708024.png[/IMG][/URL]

As you can see, in the second image the image is expanded occuping the whole width.

Thanks in advance ??