You can replace the default picture (the uninteresting grayish box) with an image of your own by clicking the Change button next to Image and selecting a different image that you have stored on your computer.
Notice that we've repositioned the image in the picture: We moved it to the right by clicking Image Right next to Layout and we made the image area 35 percent. As you work with your own business card image you'll probably want to experiment a bit with these settings to get the layout that works best for you. Be sure to try the Background Image option for Layout. With the image in the background, you'll be able to fit more text onto the card.
Note that in an EBC you can use a single image only. If you want to combine multiple elements, such as a logo and a picture (your company's logo and a picture of yourself, for example), you would do that in an image editor first, and then insert the composite image into the card.