This seems to work:

Procedure for using Google Photos:
1. Log in to your Google Photos account, navigate to the desired picture
2. Right click on it and "Open Link in New Tab"
3. In the new tab, right click on the image, select "Copy Image Location" (copying location from the URL window does not work)
4. Then go to your forum message draft, select "Img" and paste where the curser is sitting
Not sure if this helps regarding other photo hosting services. Google Photos is free and works pretty well, if you trust Google with your info.