I'm not sure what you are trying to accomplish but parenting a null to a still won't change the length of the still. That's not how parenting works. Parenting mimics the transform properties of a layer and that's all.
Are you trying to automatically adjust the in and out points of your stills? Are you trying to sequence them? I'm just not sure what you are trying to do.
I wanted to find a way of making an easier adjustment to the timing of hundreds of still images dynamically. I'm sequencing the stills but I want to play with the timings of different stills so for example, 20 stills are 3 seconds then the following 2 stills could be 12 seconds and so on.
I was thinking that if I could parent different stills to different times, that would speed things up.
I'm used to using FCP which allows me to take different attributes and copy those onto as many or as few clips as I want.
Wrong methodology. AE doesn't work that way. If you must have different timings, you need persistent "properties" and that can only be achieved by using time-remapping or effects that add property streams whose values can be copy&pasted and mangled in whatever ways you fancy. Therein lies the whole answer to your problem - use e.g. the Timewarp effect and copy&paste it with different values or use time-remapping. You could even cook up some expressions to automate this....
If you have movement on the stills you can set up expressions based on in and out points. That's what I do. I then drop all my stills in order into a comp then set the out point for all stills for the longest duration. I then set group colors for layers that I want to make different lengths, do a group select and then adjust the out points for those layers. When I'm done I sequence the layers using the keyframe assistant.
The other way I work is to use an audio track with markers for timing. I have an expression that reads the markers and sets opacity for the layers based on pairs of markers. The motion is also controlled by expressions. That's an even faster way to set up an animated slide show.
The last way I work is just to set in an out points for all layers, sequence the layers, use expressions to do the motion and then use time remapping on a pre-comp to adjust the timing of the slide show. All methods are quick and easy.
Here's a fly in bounce and then drop out fxx preset that will give you an idea of how expressions based on in and out point work for slide shows. You just drag the layer to it's resting position, apply the animatiokn preset and the layer flys in, bounces, and then drops out the bottom of the comp. Works in 2D and 3D space.
Using the Timewarp effect is just what I'm after.
Thank you Rick,
I don't have movement on this particular slideshow but using the keyframe assistant will help.
This method is definitely going to save me loads of time and allow me to make adjustments easily.