Time Machine is meant to be an ongoing backup of your files with some time history, e.g. you can go back in time and recover files that were on your computer some time ago. However, over a long enough period, Time Machine will no longer show the oldest files that were backed up, they don't stay there forever because the backup disk does eventually fill up so the oldest files are then removed.
This means you should never use Time Machine to back up files and then delete them from the source and count on them always being available on the Time Machine backup. If that is your goal, it would be better to physically move your photos or Photos library/libraries from your main drive to an external drive. (that external drive should also be separately backed up)
The files inside the Time Machine folders are not just files, they are also links, so they should never be directly accessed and modified, e.g. trying to use them to load photos directly in Photos. This could change and even corrupt your Time Machine backups.