Here I will show you how to share your images from Google Plus as 'click to enlarge' thumbnails.
Google Plus is a great place to upload photos to because, unlike other photo sharing websites, they can be displayed in a high resolution with no loss of quality or watermarks.
Open up a new tab, Go to Google+ and upload your image(s). Make sure you add them to a shared album. Go into the album and click on the photo so that it displays it in it's original size. Right click on the photo, and select 'Copy image address'.
Now you want to compose your message on the forum, so click on 'Full Editor and Preview' button. Notice an icon with an arrow and a box in the top left of the message box, as seen in the image below.
Click on this and you will see a code