Looks like the active part of the opt-out involves making the user load the following URLs (they use the script tag, but 1x1 images could do):

* http://http300.edge.ru4.com/smartserve/optout
* http://www.247realmedia.com/EN-US/opt-out.html
* http://scripts.adcentriconline.com/optout.php
* http://oo.afy11.net/Default.aspx
* http://nspmotion.com/setoptout.aspx
* http://ads.heias.de/x/opt_out.php?opt_out=true
* http://www.abmr.net/set_optout?nocache=0.1764119
* http://pbid.pro-market.net/engine?optout=$nai_optout$nocache...
* http://www.atdmt.com/CookieUpdate.aspx?option=Outredir=Cooki...
* http://optout.revsci.net/optout.aspx?a=1&p=http://www.ne...
* http://www.bizo.com/businessProfessionals/opt_out?confirm=tr...
* http://tags.bluekai.com/naioptout?nocache=0.4754892
http://s0b.bluestreak.com/ix.e?sc&t=0
* http://www.brightroll.com/privacy/opt-out/
* http://www.btbuckets.com/site/optout_submit
* http://api.choicestream.com/instr/ad/opt-out
* http://optout.collective-media.net/optout?nocache=0.3432681
* http://data.cmcore.com/privacy/privacy_handler.php?dest=http...
* http://load.exelator.com/load/OptOut.php?service=outNAI&...
* http://as.serving-sys.com/OptOut/SetOptOut.asp?OptOut=TRUE
* http://www.facilitatedigital.com/cookie_opt_out.asp
* http://fetchback.com/serve/fb/optout?nocache=0.3366319
* http://p.opt.fimserve.com/nai_opt.jsp?nocache=9.096128E-02
* http://cue.v.fwmrm.net/ad/optout
* http://optout.doubleclick.net/cgi-bin/dclk/optoutnai.pl
* http://a1.interclick.com/optOut.aspx?optOut=true&success...
* http://www.crwdcntrl.net/optout
* http://media6degrees.com/optout_.php
* http://action.mathtag.com/mm/OPTO/opt-out/OPT?oo=1
* http://mmismm.com/mm_oo.php?opt_out=&nocache=0.7812027
* http://www.nexac.com/nai_optout.php?nocache=2.282816E-02
* http://www.nextag.com/serv/main/buyer/help/AdTargetingOptOut...
* http://eu.nuggad.net/block
* http://www.openx.org/privacy?action=opt-out
* http://adserver.adtech.de/setoptoutcookie/?page=http://glb.a...
* http://www.precisionclick.com/optout_set.asp
* http://ch.questionmarket.com/safecount/nai_optout.php?nocach...
* http://notrack.adviva.net/CookieCheck.php?optThis=1
* http://ad.trafficmp.com/a/optout?s=naiz
* http://www.exponential.com/who-we-are/cookie-thankyou.jsp
* http://r.turn.com/r/optout?type=setOptOut&nocache=0.9414...
* http://www.mediaplex.com/optout_pure.php?nocache=0.9477198
* http://info.yahoo.com/nai/nai-status.html?optout
* http://ads.undertone.com/fc.php?script=optOutUser:optOutUser...
* http://optout.33across.com/api/optout/
* http://www.quantcast.com/how-we-do-it/consumer-choice/opt-ou...
* http://a.rfihub.com/oo.gif?cbust=1283182101220
* http://ads.pointroll.com/privacy/opt.asp?opt=out
* http://servedby.advertising.com/optout
* http://ad.yieldmanager.com/opt-out
* http://www.burstnet.com/cgi-bin/opt_out.cgi?nocache=3.173465...
* http://www.adbrite.com/mb/unset_targeting_cookies.php
* http://www.yumenetworks.com/optout.php
* http://www.navegg.com/en/exit
* http://rt.displaymarketplace.com/optout.html
* http://www.tattomedia.com/about_us/optout.php
* http://adinterax.com/optout.phtml
* http://ads.lucidmedia.com/clicksense/optout
* http://adshuffle.com/optOutComplete.html
* http://log.doubleverify.com/optout.aspx
* http://www.dotomi.com/opt-out-yes.html
* http://us.intellitxt.com/intellitxt/switchctxthist.asp?state...
* http://www.effectivemeasure.net/optout.php?url=http://www.ef...
* http://www.flashtalking.com/optout_cookie.php
* http://www.weborama.fr/cgi-bin/set_cookie_optout.cgi
* http://recs.richrelevance.com/rrserver/opt_out?rrnaioptout=1...
* http://www.tumri.com/opt-out-status
* http://apps.pubmatic.com/optout.jsp?action=optout
* http://api.aggregateknowledge.com/optout
* http://rotator.adjuggler.com/servlet/ajrotator/optout?action...
* http://www.admeld.com/opt-out.html
* http://ads.brand.net/optout?http://www.brand.net/index.php?o...
* http://casalemedia.com/optout/?from=optmd
* http://app.scanscout.com/ssframework/dt/userOpt.png?a=out
* https://www.outbrain.com/privacy?opt=true
* http://b.xgraph.net/placeXOO.jsp?op=add
* http://ib.adnxs.com/optout
* http://ads.bridgetrack.com/optout.htm
* http://www.proximic.com/info/optout-success.php
* http://px.owneriq.net/segment/optout
* http://info.sv.us.criteo.com/pyz/dis/optout.aspx?o=1&l=e...
* http://img.pulsemgr.com/optout?optout&action=ooatt
* http://opt.w55c.net/out?
* http://www.scorecardresearch.com/OptOut.aspx
* http://www.dapper.net/optout.html
* https://admin.valueclickmedia.com/optout/optout.html?uid=783...
* http://assets.invitemedia.com/opt-out.html
* http://www.beencounter.com/?page=no-track
* http://tracking.vindicosuite.com/Tracking/Optout/?o=1
* http://a.netmng.com/optout.php?op=optout
* http://g.adspeed.net/ad.php?do=optout
* http://privacy.traveladvertising.com/Pixel.aspx?optout=true
* http://gbid.adbuyer.com/opt-out
* http://pixel999.everesttech.net/999/optout?url=http%3a//efro...
* http://alps.adchemy.com/alps-cs/auuid/opt-out
* http://a.ctasnet.com/a?t=o&track=no&noads=all
* http://help.aol.co.uk/technical/how-do-i-opt-out-of/article/...
* http://a2.raasnet.com/a?t=o&track=no&noads=all
http://jdn.monster.com/render/nano.aspx?f530b397-a781-4f9b-8...
* http://o1.qnsr.com/w3c/optoutnow.html
* http://d.adgear.com/optout
* http://www.hurra.com/cgi-local/opt-out.cgi?opt-out=yes
* https://web.snoobi.com/en/info_optout2.php
* http://optout.yieldoptimizer.com/optout/nopt
* http://optout.mxptint.net/optout.ashx
* http://pix.esm1.net/k/optout/optedout.php
* http://t.tellapart.com/btexclude
* http://www.mixpo.com/optout/nai_optout
* http://t.mookie1.com/o/optout/cookies/mig
* http://qjex.net/optout.php
* http://spongecell.com/opt_out/cookies_clear
* http://www.lijit.com/cookie_opt_out
* http://ad.wsod.com/?view=privacy&action=optout
* http://a.triggit.com/optout
* http://www.infectiousmedia.com/index.php/test
* http://tracking.quisma.com/optout.cfs
* http://www.keewurd.com/_optout.aspx?optout=1
* http://adnetwork.buzzlogic.com/policy/cookie_opt_out
* http://www.bvmedia.ca/EN/CookieOptOut/DoOptOut.php
* http://brilig.com/opt_out_action.html
* http://predictad.com/privacy/PrivateMode.aspx
* http://www.chango.com/terms/opt_out_2
* http://ads.newtention.net/ads/privacy?cm=optout
* http://tracking.reedge.com/privacy/optout.php?do=optout&...
* http://optout.ib-ibi.com:8000/myi-behaviorOptOut.aspx
* http://a.p.mybuys.com/adn/optout?redirect=1
* http://adserver.veruta.com/optout.fcgi
* http://www.wtp101.com/opt-out
* http://rs.gwallet.com/r1/optout?optout&r1
* http://svc.pch.com/optoutsvc/OptoutService.svc/pox/Optout?st...
* http://www.convertro.com/optout-success.html?cvo_optout=all
* http://www.bnmla.com/bnmla/opt_out
* http://server.cpmstar.com/optout.aspx
* http://stage.traffiliate.com/OptPut.ashx?ReturnUrl=http://ww...
* http://ads.creative-serving.com/opt-out
* http://www2.glam.com/app/site/affiliate/nc/g-optout.act
* http://ad3.liverail.com/privacy/?optout&yes
* http://optout.cognitivematch.com/successOptout.html
* http://nxtck.com/opt_status.jsp?language=UK&choice=opt_o...
* http://www.demdex.net/demoptout.jpg?
* http://d.p-td.com/r/optout?type=setOptOut

It could be interesting if it become popular for websites to present some or all of those images to all visitors if the image size test revealed they hadn't opted out.