Thank you Admfubar. I have to use the Collect for Output function now, which works fine. But, it clutters things up a bit and I didn't have to use that in the past.
Thank you utnik. I was not aware of the Extras > Manage Images option, which revealed a "Path" problem.
For example, when my partner would open our shared working file from Dropbox and open the Extras > Manage Images, the path to all of the images pointed to the Dropbox location of those images on her C: drive, whereas the one image that I added had a path pointing the image on my hard drive, which she, of course, could not see. She could only see the red X with the image name inside the image frame.
Conversely, when my partner would make a change or addition, the path for all images (in the Extras > Manage Images window) pointed to her dropbox location when she opened the file and they pointed to my dropbox location when I opened the file, and both of us could see the images.
The question is, "Why do I overwrite her path when I add an image?" I believe it has to do with the way I have Dropbox set up.
One thing I did not mention is that I am running Scribus 1.4.8 in a Windows 10 virtual machine on my MacBook using Parallels. My Dropbox files are set up on the Mac side, but the path she sees is to my Windows VM. (I used to use Scribus on the Mac side with no problems, but when I upgraded to Catalina and now Big Sur OS, Scribus 1.4.8 no longer works.)
I plan on downloading the Dropbox app to the Windows 10 VM and add all the files to that VM and see if that fixes the path problem. Thank you utnik, Admfubar and digirew for your responses. I will post the findings of my experiment.