First you need your own Gallery, so go here if you don't already have one.

Next, Locate the image in the Gallery that you wish to share.

Scroll down to the "Picture Info" and locate the "Forum Code".

Select the content of the "Forum Code" in the field.

Right click and choose 'Copy' to copy the code to memory.

Now in your post where you want the image to appear, right click and choose 'Paste'.

Preview your post and then Post.