like putting a background image only specific page or category

You'll need to use CSS.

If you have our Simple CSS plugin (https://wordpress.org/plugins/simple-css/), you can add your CSS into the "Simple CSS" metabox while editing the specific page:

body {
    background-image: url( URL TO YOUR IMAGE );
}

Since there's no page to edit for categories, you would have to add some global CSS in "Appearance > Simple CSS" and target the catgeory with the ID.

For example:

body.category-1 {
    background-image: url( URL TO YOUR IMAGE );
}

1 being the ID of the category.

Hello Tom

Thank you
how many pixels must have the image so that it is not repeated (attached picture) the result is not what I wanted, I want you to see the image across the web, including container and widgets.

Thank you

http://www26.zippyshare.com/v/JAIlFHsb/file.html

It depends, but you can set it to cover:

body {
    background-image: url( URL TO YOUR IMAGE );
    background-size: cover;
}