Dum animation question

I am trying to achieve the following animation for a banner:

A picture fades in, then another and then another all being on the
stage simultaneously…then they all fade out and get replaced by
another bigger image. This whole thing is suppose to loop every
20 secs or so.

My question: What is the best way to achieve this chain of events?

Do I use multiple layers or place all MC’s on one layer? Do I have to use
nesting? Please give the newbie some tips.

Thanks in advance!