How to reverse these keyframes in Adobe After Effects?
If I understand correctly, you don't want to change "keyframes," you want to reverse the order of the images, so that the ones on the top of the stack appear first, followed by the ones below. 

Easiest option: 


Delete the images &amp; comp
Reimport by selecting only the 1st image, and make sure to check the box for "image sequence" this should grab all of them in the correct order &amp; make a comp of the group.


Alternative to avoid reimporting: 


Select all the layers, from top to bottom. 
Hit "Home" to move the Current Time Indicator to 0:00:00
Hit "[" to reset the start of all layers to 0:00:00
With all layers still selected, Right-Click the timeline
Choose "Keyframe Assistant"
Choose "Sequence Layers". Leave "overlap"unchecked. 
Hit Enter


The layers should now be sequenced from top to bottom, the next one starting exactly where the previous one ends.