Hello,
I think I know where the picture is from.
I'm rather certain it's not from the one
where the pokemon are trapped on the island.
The only pokemon that appeared in that episode
were Pikachu, Squirtle, Bulbasaur, Charmander,
Meowth, Ekans, Koffing, and Slowpoke which evolved
into Slowbro.  Do I sound pathetic or not?  I do
think the episode has a strong chance of coming
from the one where Ash releases his Butterfree.
Pikachu jumps from Pidgeotto to Team Rocket's 
helicopter to then zap them big time.  I would
love to have the cel where Pikachu sort of looks
evilly while on the glass window of the helicopter
just before he does electricute them.  I can still
recall that look.  And it's been a while.  It could
have come from the one with Tentacruel.  I vaguely
remember Pikachu on Pidgeotto's back, I think.  I
really can't remember real well.  Good Luck!  Bye!