Good idea, not sure why my own marketing department did not come up with it first! Gus has been thinking about plugin logos generally but you are right that it would be especially useful here.
Could even be used as the item icon (e.g., instead of a simple colored ball in list views), though this would have to be done carefully to avoid clashing with the Green Balls plugin. Some other TopLevelItem types (e.g., Folder) have special icons, though to date no Job types that I know of.
Otherwise icons do not make much of a showing in the UI—not used in the current New Item screen, and there is no obvious place in the sidebar (except perhaps Workflow Steps).