Put a track of black video the size of the sequence between the two tracks. In other words move track 2 to track 3 and black video goes on track 2.
Alternatively, keyframe the opacity of track 1.
Thanks for the suggestion, @Peru Bob. I would also like to add that ideally, altering the timing of images on track 2 shouldn't cause any later follow-up work.
For this reason, keyframing the opacity of the video on track 1 is impractical, in many cases it would result in even more work than having to change the timing of deleted sections on track 1.
Your suggestion of having an extra track with black is the lowest-effort way of dealing with this problem so far, so thanks for that. However, a method that makes later follow-up work unnecessary (such as an effect applied directly to the image clips) would be preferable.
You can always take stills over to Photoshop and resize the "canvas" to image proportions for your sequence and fill the ends with black.
Yep, thanks Neil, that's something I have done in the past and it works. However with 500 images to chose from, it's a step I would prefer to avoid.
With batchprocessing in photoshop it shouldnt take too long.
Also you could Nest all the images into a rightsized sequence with a black track in the background, and use this image-container-sequence instead of the images themselfes on the main timeline.
As an Effect you could add two drop shaddows with opacity set to 100%, a colour of your choice, softnes to 0 and distance very high. For aspect ratio lower than 16:9 (4:3 or even portrait stuff) use 90° and 270° as angle, for higher aspect ratio (cinemascope, panorama...) 0° and 180°.
Save them together as a preset will give you sth like a custom effect.
For that many images, I'd set up an action in P-shop for creating a new file to sequence specs, filling a "black" background, then re-sizing the image for top/bottom fit & centering the image l/r on top of that. Then use Image Processor to batch that action to all the files. It might take a couple hours, but then ... once you've got the action, you can tell it to go through all 500 and grab a cup o' joe. Might want to check first with say 3 widely different size/proportion images.