There are basically two ways to use backgrounds on your page. You can simply add an Image object to your Page,  select the Image to use, and select Fit to page.

A better way to go is using the Background functionality offered in the Page Layout tab.

When you click on the Background button, four options are offered as illustrated.

The advantage of using a background via the Page Layout tab is that it will always make sure that it completely covers the whole card. As you know, the card sizes vary when using a Direct to Card printer or a re-transfer printer.

With a re-transfer printer the print size of the page layout is larger and using this Background functionality makes sure that your image is always fitting the page and you will have no white lines with printing.

Choose from a one solid color.

Use a Gradient color...

Or use a background image.

Save and here you have completed adding backgrounds to your Card. 

I suggest you try as many times as possible to have better knowledge and confidence in card  issuance.