I am not seeing an "automatic" in what you describe. But, the following may be of help
What are the pixel dimensions of these 4:3 stills?
Select your Timeline file, go to Applied Effects Tab/Applied Effects Palette/Motion Panel expanded and remove the check mark next to "Constrain Proportions". That will allow you to scale the Scale Width and Scale Height separately at the monitor level or from Motion Panel expanded Scale property.
To be continued...
Great suggestion; just what I was looking for. That control allows me to scale the image so all the horizontal information displays without sidebars, yet there is no overscan of the horizontal info, as there is at the default setting. I prefer to keep "constraint proportions" checked, so the image is just cropped at top and bottom, and no aspect distortion is introduced. This is a lot easier than doing a photo edit. And the finished product looks much more pleasing. The viewer could notice that some images have sidebars, but they will never notice that some images are cropped. Thanks.
Sent from my iPad
Glad that the Motion Scale and its Constrain Proportions choices worked for you.
Thank for letting us know the outcome.