Image flashes are a form of jump scares usually used at certain points in a survival horror game to frighten the player. In most games this happens when the player does something specific like flick a switch but this may also happen by just entering a new area.
There are different types of image flashes. Some of these flashes contain images of a character in the game who is going to die or has died, other games show (a) random image(s) of monsters or other frightening creatures. Another type of image flashes try to tell bits of the story. Sometimes these aren't even meant to be frightening, for example representing a character who has lost their memory having a sudden recollection of something they had forgotten.
The Suffering shows the player images of monsters or other very hard to describe creatures. Most of the time this happens when the player does something specific like activating an electric chair. However sometimes the game shows the player disturbing images just as a way of building tension.
In the first level of First Encounter Assault Recon, the player sees an image of an beat up person. When the player nears the end of the level he sees the person of the image flash sitting on a chair beat up and barely alive.
BioShock shows the player an images of what appears to be a family. This happens between going from one section of the game to another.
The flashes seen in Halo 3 are an example of non-frightening image flashes. At various times in the singleplayer campaign, the Master Chief occasionally experiences brief flashes in which he sees distorted images of his AI companion, Cortana, who is being held captive by the Gravemind, and who is somehow communicating with him telepathically.