Trying to begin a career as a graphic designer and build a portfolio, Evan Stremke joined CrowdSpring, a Chicago-based online marketplace for creative services. No conventional help-wanted Web site, CrowdSpring is part of a trend sweeping the Web, sometimes called “crowdsourcing.” The idea is to set up an open, online competition among talent who bid for jobs doing everything from creating a corporate logo to writing blog entries. The 20-year-old student from Minnesota vies for work against stay-at-home moms, professional designers and others who want to hone their skills.