Make a note of where that photo is located, folder and file. Then just below the bottom of the area where you type your post you will find “attachments and other options” in bold. Click on that and select “choose file” then by whichever of the options is simplest, go to the file that you wrote down. You may get a “browse” option in Windows, again just navigate to where the file is, then…
Depending on software either save it, or attach it.
The file name will appear to replace “choose file”, then hit “post” and hopefully job done…