The place to start is Prof. Daniel Chandler's wondrous Semiotics for Beginners; his page of references and page of links are also quite comprehensive. The online Johns Hopkins Guide to Literary Theory and Criticism has a helpful Semiotics entry. A very nice site based on a course on the analysis of visual images at the University of Newcastle can be found here. The Semiotic Review of Books and the Cybersemiotic Institute represent a lively collection of scholars working in the area (and includes some online course material). Here's a radio program (transcribed and in RealAudio) discussing basic semiotic principles. If you are skeptical of semiotics, you might be interested in this online lecture. Winfred Noth's essay "Can Pictures Lie?" is both an intriguing analysis in its own right and a good overview of some of the literature.