My photos (on the Web) regularly get ripped off. You can fight it with digital watermarking (using special programs), or a "visual watermark": use Photoshop to layer a graphic like "©2009 Tommaso Dorigo" (opacity = 20%). It's practically impossible to edit out; they would have to reproduce your original diagram. You also need to put on your slides (all of them, including text): "©2009 Tommaso Dorigo, All Rights Reserved". I understand some journals take ownership of published papers (ripoff!!), which leads to the unusual situation that the original author could get sued by the publisher (if author puts the paper on his website).

There are 2 sorts of copyright infringement: accidental & willful. The former is where the offender didn't know (it might have floated around amongs several parties). "receiving stolen property". The latter is where the offender did it intentionally, & by not giving credit to original author/artist -- he is an thief. Everybody hates thieves.

My experience in unacknowledgement of intellectual property:
During my PhD research (where I made a breakthrough concept, when the field was hopelessly stuck), my idea was stolen by a collaborator. He never acknowledged it was my idea. Naturally, his arrogance led to a mistake in that same paper (which I found, & the entire algorithm was invalidated), which was reported to the journal. They had to acknowledge me for finding the error, which was an embarassment. To this day, that fouled-up algorithm is used by texts & courses in major universities (I can point to the Caltech course URL). That same breakthrough idea of mine was used to solve the (n+1) order problem, which these same clowns spent an ENTIRE SUMMER (Swiss Federal Inst) fruitlessly trying to solve. They took my idea, FRAMED IT in their method (difficult, less intuitive than mine), & used the adjective "evidently" in their derivation!! TOTAL HOGWASH. My geometrical method (obvious to a high school student) leads to a derivation of constraint equations in 15 secs, while these clowns (including my PhD advisor) frustrated themselves an entire summer in Europe. This was the beginning of a campaign against me (newcomer to the field), which led to journal submissions being blocked. They even sabotaged my employment opportunities after graduation (IBM TJ Watson Research Lab).

If I were you, I would find out if this guy ripped you off intentionally. If so, you need to put a stop to this (& other ripoffs), otherwise the problem will get worse. Look at the El Naschie scandal, that's just crazy stupid. (which leads to Bee's presentation at Google "Why Academia Sucks"). That's the problem with Academia, this stuff goes on ALL THE TIME without any time of Enforcment (as part of the Rules/Enforcement model). Researchers (preferably part of a team), need an enforcer to go out & stop the "bad guys". Otherwise, you have chaos. That's how professional Sports keep things "in order". Baseball & Hockey are famous for retaliatory strikes, to "balance things out".