I have learned from a video tutorial (Flash CS5 in the Family 23: Banner Ad Part 1)
to use the F5 key to set the length of a time line of a particular image.
When using the F5 key on the MAC keyboard, it will only allow for the audio to be worked with.
I've tried Apple+F5, Option+F5, Ctrl+F5 etc.
What am I missing?
click edit/keyboard shortcuts and create your own shortcuts or, even easier, right click over the timeline (or anything else in flash) to see a context menu that will let you do what you want.