Unfortunately I know nothing of Ajax or PhP but I'm a quick learner...
Yes I actually created 150,003 unique images for testing my project. The final images will be supplied by someone else.
I used After Effects. Created a "video" that was 100 x 100 pixels and added a Timecode effect. Video timecode tracks every single frame with a unique number. I set the timecode effect to display units as frame count instead of Hours Minutes Seconds Frames and it counts from 0-50,000 one number per frame. I output the "video" from After Effects as a .PNG sequence, one image per frame. Now I have 50,001 images. I tinted the timecode 3 different colors to match the colors I'm using to debug and I ended up with 150,003 images. The whole process took about three hours of me playing Injustice while it cranked away.
Bonus: All my emitted images are sequential numbers so its easy to know if things are really working well. I can tell right away if my randomizers are working or not, or if I am losing things someplace with skipped numbers, etc.
I guess I know what I'm doing during down time at work tomorrow... reading up on Ajax and php!!
Thanks for pointing me in the direction.