well, Tony Moore was the first artist. I think he drew for the first six issues, what us comics enthusiasts call "arcs" (because it contains one story arc). I think Tony is one of the best artists in the industry, but I do not think he was right for this book. he is a little bit too clean in his approach, but to give him respect I have often compared his sketches to "crack." yes, I am a serious comic book nerd, with no help in sight
Charlie Adlard, a Brit, takes control of the series past the first six issues - and I love him! although perhaps not as artistically gifted as Moore, Adlard is gritty, dark, and horrific. I think his style lends perfectly to the image that TWD is trying to go for
since the book is in black and white, I think that the newer artist is more useful
if you have been thinking that the book's style changes in-between the first arc or also other than the two artists, I would say that it was an artists trying to get the feel down for the book and changing his approach slightly. if you read other comics that have been continuing for years and years, you will notice that the artist improves and has gotten better and changed his style slightly within those years. it's a learning process, and they are always trying to improve. another reason why I love this median