In the Multiple Record Layout dialog, click the Options tab choose Content Placement options:
Content placement options
The following options appear in the Content Placement Options dialog box:
Fit Images Proportionally
Maintains the image’s aspect ratio but scales the image to fit within the frame (default).
Fit Images To Frames
Scales the image so that its aspect ratio is identical to the frame’s aspect ratio.
Fit Frames To Images
Preserves the image size but adjusts the frame size to match it.
Preserve Frame And Image Sizes
Places the image at its intrinsic size into the frame, aligned to the upper-left corner of the frame. The image is cropped if too large for the frame.
Fill Frames Proportionally
Places the image so that its height or width fills the frame, leaving the rest of the image cropped.
Center In Frame
Aligns the center of the image to the center of the frame.
Steve's answer is referring to the alignment of variable images during a merge. A visual reference can be found here: https://colecandoo.files.wordpress.com/2012/08/framefit.pdf
Looking at your post, it appears that you want the merged multiple record results themselves (not just the images) to appear aligned based on the centre of your page. This is controlled in the Multiple Record Layout tab of the Create Merged Document dialog box during a data merge, specifically the margins panel.
Yes, it always merges with the first graphic (and subsequent graphics based upon this) to the top left.
I'm guessing this centering is being done for imposition purposes? If so, there may be better ways to accomplish this task.