Store photos from external SSD on Time machine

Hi.

I keep all my photos on an external SSD. I also save documents regularly on my Time Machine. Somehow I got information that I should not back up photos from the SSD to time machine (a regular external disk). Instead of biking up photos with Tim Machine I copy them to another disk.

A bit silly, I think. So what is true here? Do I have to do it the hard way?

Nils