Pearlsasinger
Up in the clouds
Well I was always taught to say "Whoever and I are doing whatever" You don't say "Me and whoever are doing whatever" Or "Whoever and me and doing whatever"
You were taught quite correctly.
You should indeed say "Whoever and I are doing whatever" because you should say "I am doing whatever". In those sentences,"I" is the subject of the sentence. In other words "I" is the one that the verb refers to.
In the caption to your son's picture "This" is the subject. In any instance where you would use "I" if only one person were doing whatever, then you use "I", even if at the end of a list of hundreds of names. However, if you would normally use me, as in "This is me", then me is correct, even if at the end of a long list of names.