Hi Albert.
You can do this easily using the "Align and Distribute" palette.
1. Select the "Windows" menu and, from that, "Align and Distribute". A new window/palette should appear.
2. Select your image frame (or whatever you want to align).
3. Go to the "Align and Distribute" palette and open the "Relative to" drop-down menu selecting "Page".
4. Click the "Center on Vertical axis" icon (middle of the top row of icons, hovering over the icon will let you see the tool tips).
Your frame should now be be centred across the horizontal of the page.
You can use combinations of different "Relative to" settings and the various different icons to achieve different effects.
I hope this helps.
Garry.