Start with these images from this link the WebPageTest provides, you have several that need to be optimized and the site will do that for you.
Images at WebPageTest
Anywhere you see in bold, "Not Progressive" click the link that says "Analyze JPEG" and the site will generate a fully optimized image for you. (The third one it shows) Download that to your computer, re-name it to match the file name it is on your server, and upload to same location allowing overwrite.
Also where you see the grades at the top? Click on the F ones and the site will tell you what the problem is and how to fix.
I test with IE 10 using a fast cable connection, to keep results consistent and to make sure I am testing to latest standards of the new browsers:
Here's your site on that test, this morning:
WebPagetest Test Result - Dulles : nirc.co.uk - 02/26/14 16:41:56
Another thing you can do is get rid of the gallery thumbs in sidebar for visitors. Make it so that's only seen when logged in. Also save alot of db queries and calls by making the top 5 stats visible to logged in members only as well. Compensate for the length of the sidebar by displaying more of the "latest threads." Google likes that content much better than thumbnail images anyway.