This is still really annoying, 5K imac can't even render out to play a 5k video file. Going through the extra step to down res all of these photos is just plain stupid just because apple doesn't feel like fully supporting Quicktime 7 Pro.
QT 7 a 20-year old application originally developed to support the display of postage stamp sized videos. It was not designed to specifically handle 2K, 4K, 5K, 8K or other UHD content and is no longer under development by Apple since Apple is now devoting its time and effort to the active development of QT X—a completely different and more modern multimedia structure that supports both program and transport stream content in ultra-high display densities.
I have nearly 250,000 photos that you are asking me to resize to a smaller file size, and since I always keep the originals, I will have an extra 250,000 duplicates sitting on my drive. Jon, Thank you for answering these questions, but this is not a solution, just a work around.
Depending on what software you use to manage your photos, you may be able to batch process your photos to new target resolutions—which is what I do in Aperture. In addition to the resolution limits on QT 7 Pro already mentioned, there is also a limitation on the number of photos that can be sequenced to a single clip—i.e., 2^16-1 or 65,535 "photo frames" in a single sequence (assuming this has not been changed since I last tested this limit). As to this not being a solution, you can always search the Internet to see if there are any third-party applications better meet your current needs.