most of this is just an issue of using the . syntax.

there's 4 ways to use it:

_root.
_root. refers to the topest timeline. you'd use _root.something to call something defined in index.swf from an included swf, or a deeply attached movie clip.

_parent.
_parents goes up one timeline level. IE _root..lvl1.lvl2. in inside lvl2 you need to access a variable defined within lvl1, you'd just use _parent.varname

this.
this just refers to the current timeline - this.gotoAndPlay(1);

instancename.
if you set a movie clip called "preview" to load your images into, on click a button would say something like previews.gotoAndPlay(2), which will start playing the previews timeline at frame 2.