Hi Cezary:
You could click on the background and then open the properties palette at the top right. There you can see the width of the page in pixels.
Then you place your image, click on it, and get its width from the property palette.
Subtract the width of the image from the width of the page, then divide by 2.
Enter the result into the image properties - "Left".
Example:
Your page is 900 pixels wide.
You want to center an image that is 600 pixels wide.
(900-600)/2 = 150
The Left positioning of your image will be "150px"