My apologies for the late reply but I've only just registered.
This may not be exactly what you were after but you can get an image centred in an image frame by doing the following:
1. Create the image frame.
2. On the image frame, Right-click, then Get image…
3. On the image frame, Right-click, then Adjust Frame to Image.
4. Select the image frame.
5. In the Properties window (F2 if it's not open), select the Image tab and then select the Free Scaling radio button.
6. Drag all of the edges of the image frame away from the image.
You'll need to manually make sure you drag each side by the same amount by using the numbers on the "tool tip" that's displayed when you drag as a guide.
Obviously it's not an exact science but do you really need something to be exactly centred? Surely it's just got to look right on the screen/page?
P.S. I'm assuming that you want the image centred with space around it so that text doesn't come too near the image.
If that's the case you could also try the following:
1. Create a rectangle shape the size you want to space to be.
2. Select the shape and in the Properties window, set the line colour - in the Colours tab - to None (so the shape is "invisible").
3. Still in the Properties window, set the Text Flow to use Frame Shape or Bounding Box (depending on what you want).
4. Create an image frame and get the image how you want it.
5. Open the Align and Distribute window (from the Windows menu).
6. Drag-select the "invisible" shape and the image together.
7. Using the Align and Distribute window select both the Horizontal and Vertical centering icons.
8. Group the shape and image so you can move them together.
I hope some of the above helps.