Well, I'm not sure as to how it would be insulting to Wiccans, as Wiccans are not a group associated with any Witches that were stereotyped that way. Wicca is a fairly new religion and holds no ties to the stereotypical flying, green faced witch.
Now, as a practicing non-Wiccan Witch, I can say that I hold no insult to the stereotypical Witch. I think it's a misrepresentation of Witches, but not insulting. It's just common place.
I'm not sure where the green face originated from (as far back as I know of the green faced Witch is from The Wizard of Oz). The flying broom thing is a misunderstanding. Many, many, many moons ago, Pagans would celebrate the harvests around large bonfires. To ensure a prosperous harvest, many Pagans would straddle a broom and gallop around the fire or field. Some other practices involving brooms included a tradition in which they would jump while straddling the broom. The higher the jump, the higher they believed their crops would grow. Some people witnessing these acts may have misunderstood what they were doing and thus believed they were trying to fly.
As we all know...no one can fly unless assisted by either an airplane, hang-glider, parachute, or the like.