not sure how to send a photo without an email.
You can start a 'private conversation' by clicking someone's username:
Then select "Start conversation"
You can then create a message that's similar to an email. To include photos, you can simply drag & drop them into the message on your desktop computer.
On a phone, it's also possible to include photos by using the "attach files" button:
Then navigate to the photo(s) you want to include.
If you're posting from your phone and the photos are too big to be included, google for "resize images on iphone" or "resize images on android phone", and similar terms for desktop Windows/MacOS.
Hope this helps!