I get same result with Avast, Norton and AdBlock Plus enaged, Greasemonkey disabled: first thumbnail requires 'click to see image'. After clicking i get one pop-under. All imagetwist thumbs after open without opening another ad. This has always been our agreement with image hosts. They are permitted to serve one pop-up ad to the same IP during 24 hours.
You might try using ShowJustImage3 script to block ads on all image host pages. Get it
here. Greasemonkey for Firefox, Tampermonkey for Chrome must be enabled to download scripts.