The between subject part would have to be done by assigning different sets of images to different condition IDs. That has to be done by you because MediaLab doesn't randomly assign subjects to between subject conditions automatically (for various reason outlined in the users guide). However, given a set of different distribution images, you could have MediaLab randomly pick one from each set. See (esp. the advanced hints):

http://www.empirisoft.com/medialab/help/queskipto.htm