How can I stop people stealing my photos?

[Andi]

If you put them in a public place....
what do you expect

in fact, many of the T&C's of these sites
say that by publishing them on their site, THEY own the image (by your consent and action)

If you wanted them private,
and to preserve the Copyright, you should have kept them private
and when sharing them, remind people that they were yours and not to be shared beyond the people you were sharing them with.

In theory,
the Photographer is considered the Artist, and owns the copyright.

But if you are photographing another Art work,
then you are breaching their copyright.
It could be argued that the creating of the dish (that you photographed) is the original artwork.

Likewise, that you took the photos while an employee, of work items, means that they belong to your employer anyway (as part of being employed).

The situation you describe has many wrinkles
not helped by your public publication.
Your employers could also come after you...