Image presentation time is bounded by two factors: the visual persistence effect  and subject strategy development.
The visual persistence effect is found when we look at an object for a short period of time: for a short period of time after a visual stimulus has been removed we can recall nearly all of the detail of the stimulus, but after that time recall becomes much worst. That is to say that we have a short term visual buffer. The persistence effect is typically measured at around 250ms. If the presentation time and the period of the delay between presentation was lower than 1000ms then the persistence effect would represent a more significant part of the subjects recall mechanism, producing unwanted and uncontrolled for effects.
In pre-experiment trials subjects reported that they developed strategies to determine image quality difference when the image was displayed for more than about a second - subjects probably do develop strategies however the effect is less marked with a shorter presentation time.
It was therefore decided that a presentation time of 1000ms for each image and delay of 1000ms between images combined with a random presentation order would be the best Presentation criteria for the experiment.