I currently don't know any place that has good pictures from the Animatrix, but what you could do is:
Download the 'movies' (parts of the series) and play them in Quicktime (duh) then skip to the frame(s) that you want.
Pres CRTL+C. The picture has been copied.
Then 'paste' it in a program like 'paint shop pro' or just 'paint' (whatever you use).
And there you have the picture that you have copied.
I don't know if this really helps but, have phun with it :]