The trick is that the image must be in a shared album, otherwise people who are not logged into Google as you will not be able to see it. Do the following:
- Put the photo into a shared album in Google. This is good to do anyway to keep things organized. Make sure the album is marked as shared; if not go to its Options to turn on sharing.
- Open the photo from within the shared album.
- Right click on the photo and select Copy Image Location or similar, depending on your browser. If your device has a touch screen and no mouse, try long-press instead of right click (works for me with Firefox but not Chrome).
- In your forum posting click the Insert Image icon and then Ctrl-V (press the Ctrl and V keys together on your keyboard) to paste in the URL that you copied above. It will be very long! Or with a tablet/phone, long-press, position the cursor and then Paste.
- Look at the end of the URL that you pasted in, and change the image size width/height values after “w” and/or “h” before the “-no”. For example change "=w1317-h741-no" to "=w800-no". This will make the size more suitable for forum display.