Some hints since I can see the participation is a bit lower than expected so far:
1) Focus on sportsbetting and casino, stay away from poker.
2) You can use objects to make an icon but do not make it too much.
3) Betting companies tend to have wordmark type logos.
Most of the online bookmakers you will see today are the online versions of old British, Austrian/German or Scandinavian bookmakers such as William Hill (www.williamhill.com), Paddypower (www.paddypower.com), Ladbrokes (www.ladbrokes.com), Unibet (www.unibet.com), Bwin (www.bwin.com), Admiral (www.admiral.at) Betvictor (www.betvictor.com), Interwetten (www.interwetten.com), PAF (www.paf.com), Sportingbet (www.sportingbet.com), StanJames (www.stanjames.com), Expekt (www.expekt.com).
There are younger bookmakers who entered the business with .com revolution: Bet365 (www.bet365.com), Youwin (www.youwin.com), Pinnacle (www.pinnaclesports.com), Betfair (www.betfair.com), Bet-at-home (www.Bet-at-home.com), Tipico (www.tipico.com).
Finally, there are Asians: SBObet (www.sbobet.com), 12bet (www.12bet.com), 188bet (www.188bet.com), dafabet (www.dafabet.com).
As you can see, most of these companies have logos with wordmark but some of them have an icon that makes people remember the brand. My favourites are certainly bet365, bwin and 118bet.