Hi.
As noted by pezlab you could solve this issue by utilizing key presses in place of mouse clicks. However, if you need this experiment to be conducted via a mouse, below is an alternative to your current design features within DirectRT.

In DirectRT, after the presentation of both images and after a participant clicks on an image, immediately following that screen you can insert a new screen that requests the participant to center the mouse, for example, with a centered "X" and an instruction to place the cursor on the X, and then press the space bar to continue to the next image choice trial...

Even if you could automatically center the mouse after each image choice trial, you would still be uncertain of subtle off-centering movements made by each participant, because simply holding a mouse causes it to move slightly or a lot.

Note: In the MediaLab forum, version 2008, a similar thread on this issue concluded that no such mouse centering option existed.
FYI:
http://support.empirisoft.com/showthread.php?t=1364