One of the most powerful images in The Scarlet Ibis short story is the first, largest and most obvious one - the ibis itself, or rather, it's colour. Red is the colour of blood and as such has become associated with death. It is also the color of danger, which in the case of this story could be seen as a warning of things to come - look out for clues the author gives the reader about further developments in the story.
The color red has been popular in many stories and plays to denote negative things such as death or murder - remember the red in Macbeth. The plight of the disability is also a theme, and red is often used as an image of anger. Creatures that are red such as chestnut horses - or even red-haired people are often said to be fiery-tempered.