Ulric Neisser came up with the term iconic memory to describe the visual impressions that your mind gets from your eyes. This doesn't refer to the pictures your store in your long term memory; iconic memory includes nearly all visual stimuli you receive and lasts for only 1/4 of a second. During this small amount of time your short-term memory will pick out the useful bits and save them, and the rest are discarded.
George Sperling developed a test to find the size of people's iconic memory. He used a card with 12 letters on it (4x3). The card was flashed on the screen for a fraction of a second, and immediately afterwards the subject would hear a tone--either high, low, or in between. If it was the high tone, they were to report the top line, the low tone was for the bottom line, and you can figure out the rest on your own. Subjects could remember, on average, between 3 and 4 of the letters from a line, and since they didn't know which line to focus on beforehand, they must have had about 9 or 10 of the 12 items in their sensory register. If, however, the subjects were asked to report all 12 items, they could usually come up with only 4 out of the 12. Their iconic memory faded before they could report all the letters, and even before they could enter the letters into their short-term memory.
You sensory memory also includes Echoic Memory.