Click on the photo you want to share. At the bottom right corner, you'll see a star, an arrow pointing to the right and an arrow pointing down onto a flat line. Click the arrow pointing down. Now click on "View All Sizes". That will take you to a page with your image, and a selection of different sizes to display. Click on the size you want to display. That size image will appear. Right click on the image and select "Copy Image Location".
Come back here to the forum, open up a new post and click on the "Image" icon. Hit "CTRL+V" and the address of your photo will appear within the "IMG] brackets.
Press "post" and your image should be posted...