Usually, you must create paterns in your line to minimize the "jag" of your line. It's difficult to explain without any exemple, so I'm leaving you a link where you will be able to understand what I mean. This "rule" is, with the shadow and light, probably the most hard to get at first, but with practice, you will do it without even thinking.
Note that it's not a rule made of stone, but if you stick to it at first, you will be able to go against it after time and still keep a visual harmony in your drawing.
Usualy, I'm doing a silhouette of my character (or piece) before going into the detail. If the silhouette seems jaggy or uncool, the final picture will look jaggy and uncool, even if it's full colored and animated.
N.B. If you red my bio, you know that english is not my native language, so if you missed some part, don't be shy to tell me and I'll try to correct what I said
Hi! dude, I like your headstone. Maybe the only comment I have is that your black outline is not regular. It's the only thing that catch my eye at first sight.
By the way, do you have a full license of GraphicGale, if not, you should help the developper and buy a full license. It's around 20$ for the license. Anyway, I use it everytime, I like it a lot too, very useful and friendly.
Anyway, continue your good work :)