Maybe a caricature of a bull (to signify bull market optimism) with something else to evoke the stock market (a chart pattern? dollar signs?). Another idea is to do a cowboy which is a play on words since my blog is entitled Kaoboy Musings. When it comes to stocks, green signifies up and red signifies down. The logo or bull caricature can be an color but maybe the background should have red/green in it to symbolize stock movements.