I've had to explain to several employers along the way that "Teaching" is not a part of my job description or an addition to my job description that I will accept. Often I find that employers assume that they can make small changes without anyone being annoyed enough to say anything. I've also had to do the same with VisualBasic and Perl. I don't list them on my resume not because I don't know them, but because the last thing I ever want to do is work in VisualBasic and I consider having to edit other peoples Perl code to be a little worse than having teeth pulled.
I've had employers who were very understanding of this. I've had others who simply didn't understand why I would place this kinds of restriction upon them. To this I remind them that in the interview I pointed out that I'm a poor teacher and that although I know several computer languages I'm trying to focus only on (fill in the name here according to the interview). That if they had different expectations of me than what was expressly stated in the interview and on the resume it was a serious issue and one that needed to be addressed imidiately.
I don't teeach people how to write HTML code, I don't teach people how to write javascript. If you want to learn I can suggest several books and schools that will teach you correctly.
In your case, I would suggest a meeting with your boss as quickly as possible. Generally I try to explain that although you will finish the task as assigned this time (always agree to do the work first), you consider it a bit of a problem moving forward. Remind them that just because your a good cook doesn't mean that they can assign cooking lunch for everyone as one of your duties. You view graphic design in a similar fashion, although part of your skill set and an asset to the company it is not actually part of the job your trying to do, or that was agreed upon in the hiring process.
