Newbie - how to modify a picture

Hello,
I am learning with Flash 5 (I got PC with Flash 5 installed). I copied
a cartoon to Flash and tried to modify it - but even the picture is
not locked - I cannot modify it. When I add a line - I see a change,
but the moment I release a mouth button, my line disappears and
original line pops up.
My question - how to modify a picture I "pasted" to Flash?

Thanks,

Zalek