Your best option is to get away from the timeline completely, put all your image links in an xml file, load it in, then use timers to pick a certain length of time before it changes.
However, this may require quite a lot of learning, depending on your familiarity with AS3.
I'd recommend playing around with Caurina Tweener too, which gives you wonderful control over your AS3 driven tweens (rather than the timeline ones).
tweener - Project Hosting on Google Code
It's simple to set up- put the folder in your working flash directory. Import with:
Code:
import caurina.transitions.Tweener;
And then put in tweens like so:
Code:
Tweener.addTween(myNewImage,{alpha:1, time:1, transition:"linear"});
(that would fade to full alpha in one second).
Alternatively you could just download or buy one of the many image galleries out there which do this job proficiently, are customisable, and could save you a lot of time!