Honestly, Wikipedia has always been a pretty good starting point. They'll tend to have links to review articles with some nice diagramatic representations.
PharmGKB, a genomics website, also has some excellent pictures and explanations, but only when the drug has a decent amount of genomic interaction (the diagram for warfarin is crazy). You might also want to try the different drugs' websites, they tend to have videos and the like in the "Healthcare Professionals" section...from what I remember, Zetia and Plavix have pretty good ones.