I understand that creating a new property might not be the best way to go about this.
I may be wrong, but the way I have implemented it would require a constant amount of time to lookup each name.
I am aware that there is a list of costumes but iterating through the list every time a new costume is to be added is linear time and gets slower and slower.

Perhaps making this a property of the doScreenshot function rather than the SpriteMorph would be better?