Hi
Few weeks ago, i had some intresting experience with "scary" Is's...
The most uncommon ones, ware sudden IS's wich occured durring school few times, those images ware and are :P indescribeable, something like nithmare of nighmares , evil essence ?.. smth like this, THose ware Huuuuuge really multi dimensional and there ware no "static" shapes...
it was something like a flesh made world, with some ugly not ?identyfied? creatures...
Those IS's ware really short few seconds ...
but they are horrible.......................
SOmetimes on the beginnig of an Image Stream i see "bad" pictures but, i just relax , take a deep breath and start another IS... :P
What i think is that violent IS's are appearing as a reaction to enviroment, media etc.
Visualizing Human faces is common, and normal , i get my first pretty good face visualisation after good training :P
cant say how many hours... it will come with time.U may try delta/theta IS's wich are helpfull. I have an IDea!! Maybe use some happy music to change the mood of your Is's !
That maybe good. Also try doing your image streams in some really safe place.
If i undestood right what AlexK said, try something like this:
Try to make those ugly images smaller and smaller with every second
move them around
and try to imagine a positiv image at the same time and make it bigger and bigger until the positive image "locks" and the negative dissapears 
Hihi you can bollive me or not but You can seduce woman by moving their pictures around ^^
HAfe fun
GoN
