Start a weblog. Place a downloadable image on it in a common format (SVG, PNG, Gimp, PhotoShop) placed under the Creative Commons Atrribution-Sharealike license. Let people modify and re-upload the image using the comments mechanism (censor uploads that don’t preview, offer help if people get this wrong). Collaborative art.
After a while upload a new image as a new topic and begin the process again. Use elements from the previous work or make a call for new images. Encourage an accompanying discourse (or at least discussion). Take the work somewhere. Make shared objectives.
Art & Language’s 1970s Indexes provide a good historical counter-example to the whimsy of Exquisite Corpses and Mail Art for collaborative artwork. The net can be studio and gallery simultaneously. Work can be done this way. More, it should be done this way. This is culturally urgent. The relations of production, distribution and consumption as well as the creation and extraction of value must be changed.