On ebay, right click on the picture you want to post and click "Copy image address" (or 'copy image location', or similar). You'll get a really long URL ending in .jpg in your clipboard.
When writing your reply on the forum, click the 'image' button (it's the one that looks like a green square). A little window pops up. Paste your ebay picture URL in here. The eBay image should now be in the reply window. Don't worry too much if it's massive, the forum resizes them to fit.
Now to make it clickable, nip back to eBay and copy the address of the auction page. Come back to your new reply and click the image once. This should highlight it. Now click the little chain icon and paste the auction page address in there. This will make the picture a link to the auction. If this step doesn't work for you, just pasting the address in the reply is fine too.