Supervisor used graphic without crediting me The following situation occurred:


I was asked to modify a figure (originally published in a paper) to be used in a non-further-determined publication by my professor. 
I did substantially modify the figure: I redrew the figure from scratch in a vector program, added colours and decoration and made it generally more attractive.
Now, half a year later this same figure or graphic appears in a book and just holds the small remark © Name of professor. Neither my name nor a reference to the original figure is given.
I checked the general acknowledgements of the book and there is no mention of my name.


My question is: How to proceed from here? Naturally I don't want to deteriorate the relationship with my employer but I need to raise this issue, especially so that it does not happen again in the future (my professor asks me on a regular basis to draw graphics for him). Should I insist that my name should somehow be included (second edition of the book, online errata etc.)? What would the copyright implications be for a) not mentioning the creator of the graphic (me) and b) not referencing the original work?

Some context:
I work as a research assistant and I am not employed primarily to do these drawings though - when requested - I usually do them in my work time. The book in which the figure appeared is the output of a research institution (though clearly directed at the general public) and I am not sure if the book is to be sold, i.e. if it could be considered commercial use.

Edit: I may not have explained myself well. In my opinion the resulting image is not a mere modification of a figure but a piece of work that requires creativity and year-long practices with a graphics program.
How to proceed from here?

You don't do anything. You did something small your supervisor asked for, and he used the figure that you made for him. 

It's very unusual for figures to be labeled with the name of the actual person that made it, and it would be a lot to expect this without asking / demanding this beforehand. Take the closest textbook and you'll find that indeed,  a lot of figures will be copied from publications and references are included. However, there will also be custom-made figures that don't have any note. And you can be quite sure these figures are not drawn by the authors themselves, but by their students or employees. 

There would probably also be no copyright implication, as this sounds like you just re-created some schematic figure and you just copied the idea, not the work itself.