You left out some very important details such as the exact model of the Mac and version of macOS being used.
Are these photos being accessed from the Photos app, or some other app?
Are the photos stored on the boot drive or some other location?
Are you syncing these photos to the cloud with iCloud or some other cloud file syncing service?
Try rebooting the computer to see if that makes any difference since a simple reboot can fix odd issues. If a simple reboot does not help, then try booting into Safe Mode to see if that makes any difference.
You may also want to run the third party app EtreCheck and post the complete report here so we can examine it for possible clues.
You may also want to run DriveDx (free trial period) to check the health of the internal drive(s). Post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. For a system that old, I would be concerned about whether the hard drive is worn out or failing.
Assuming the drive is healthy, you can also try running Disk Utility First Aid on the hidden Container to check for file system issues. Within Disk Utility you may need to click "View" and select "Show All Devices" before the hidden Container on the left pane of Disk Utility. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are errors listed, then run First Aid again until the errors are gone. If the errors remain, then you need to run First Aid while booted into Recovery Mode.
FYI, You should not be worried about the data on the drive since that data should be part of your backup regimen if that data is important. People should always have frequent and regular backups of their computer and all external media (including the cloud) which contains important & unique data. Apple includes Time Machine with macOS to make this process very simple (not sure about the cloud data though). People can also use other backup options as well, or a combination of backup options. Of course this won't help you now, but it should minimize the chances of needing difficult troubleshooting procedures to access data on a corrupted volume....and hopefully it will be notice for others reading this thread.