How much a picture would you see if you only saw it for half a second?

This is a question more suited to a sensory psychology forum. A half a second (500 msec) is a little longer than an eye blink, which is 300-400 msec. Hope that your viewer was not blinking at the time. If they weren't then your presentation is quite literally in the "blink of an eye". How much would you get of it? It depends on the content, which means how much perceptual processing your brain has to do to perceive it. Something simple like a geometric shape or a word would be picked up quite well. A more complex scene, however, might leave an impression and the viewer could actually be wrong.

There is a circumstance where the brain will store a snapshot of a flash scene for a few seconds, long enough to make out more of it. However, it can only be duplicated in a darkened laboratory and if the subject's eyes remain perfectly still after the display.

That would probably depend on what the picture was of, how detailed it was, how big it was and whether you were expecting to be given half a second to look at or it was just seen in passing.

odd question... You would just see a glance at the picture. a general subject of what the picture was

You'd still see the whole picture.