Regarding how to randomizing list with having equal probability

[deanhystad]

Each of the images are eventually presented 50 times.

Unfortunately that is impossible given the constraints. 9 images per set * 150 sets does not equal 24 images * 50 times. The math does work out for 8 images per set.

Either the set size has to be 8 or the requirement is "Each of the images are eventually presented at least 50 times." If the set size is 9, the problem is tricky. You cannot randomly shuffle and process sequentially because the number of images is not evenly divisible by the set size.