The picture of the adult Skeeter, I'm told, was from "You've Still Got Me", per Muppet Wiki.
Also, I have to say that I have to agree with Karen Prell doing Skeeter, given that both Skeeter and Red Fraggle have athletic personalities (if you notice in one of my fan-fic sketches, I described our proposed adult Skeeter's voice as a "Red Fraggle-type"). ;-)
On the other hand, if we had a guy doing Skeeter, I'd picture Marty Robinson or even Kevin Clash doing her (I've often thought of her adult voice being a cross between Elmo's and Benny Rabbit's).