First, dumb question: do you have pictures uploaded to a hosting site? If so, right click on your picture. Click on Properties. Right click on the URL to highlight it. Right click again and Copy. Click Okay. Then come to the discussion forum topic (or PM) and click on Image. Right click on the higlighted http:/. Click Paste.
For a link, follow the same procedure as you would for a picture. That is, right click on a link, Properties, highlight, right click and Copy, Okay, Click http:/. Do you want to put it the link in the Growlist Forum or in your signature?