In Flickr click on the photo you want to show from your 'photostream' account.
Very slightly above the image you will have some grey options click 'all sizes'
select either medium or small for forum use. Now when you scroll down past the image you will see two text boxes filled with geek speak, ignore the first box and copy the box that says 'Grab the photo's URL'
Now on this forum, when you want to post you will see alot of options above the text box(the box you type into), one will look like a mountain range with a sun, click this and paste the URL here, Bingo, your image will now be posted, click 'preview' to check