Plot, how would originality work? Just a basic "ten-or-zero" score based on whether the sprite is original or a recolor/edit?
Adherence to theme would refer to the sprite's... Adherence to the theme. However, it's rate would be kinda determined by originality. You see, one can adhere to a theme, while not following its rules to it's literal extreme. That's where originality comes in. One can be original WHILE staying true to the theme, through interpretation. Suppose the contest calls for you to create a Castlevania boss based on a Halloween character. I could create a pumpkin headed man with a shotel, and it'd adhere to the theme to perfection, but would lose points in originality. On the other hand, I could create a boss based on candies, a monster made of stolen candies from sad children. It could lose a little on adherence but earn a lot on originality.
Originality, in my humble opinion, would automatically be a zero if the artist simply recolored an existing sprite (UNLESS the contest calls for a recolor). Frankensprites could earn some originality points, since recombining sprites in a cohese shape is complicated, but would still not get many originality points.
By "originality" I also mean "creativity". However, creativity by itself tells you to do whatever the hell you want while using artsy means, while originality tells you to twist an existing concept in a new creative way, while referring to the original concept. The contests here will call for art based on Castlevania most of the time (the original concept), thus I though "Creativity" is not exactly the correct term to use as a judging category, even though it DOES work. I suppose there will be contests calling for other sprites unrelated to Castlevania, and in this case, "Creativity" could be added to the judging categories, or outright replace "Originality". But for Castlevania based sprites, I though Originality is more accurate.
I also mentioned Skill. Skill is the most technical, since it requires the person to have precision, be detalist, have a hand in composition, sharpness, shading and overall aesthetics. One can be "original" or "creative", but still must maintain a level of skill to pull these off successfully enough to earn an award. And that's why I though about the mods awarding people who can pull off originality but not skill, with the Alloy Pot Roast, since I personally feel that it's kinda bad to not prize originality when it lacks skill. Someone skilled, but not original can simply earn the bronze award. I did not use "Quality" because quality is a by-product of skill. If you're skilled, your job has automatic quality to it. It's hard to judge "Quality" because it refers to something that doesn't really carry technical weight to it, and is more subjective. My Golden Bat might look "quality" to me, but look like crap to you. Thus "skill" avoids subjective opinions, while helping the judge to quantificate/explain how "qualitous" the job is through technicalities, I think.
It's "easy" to draw a fucking awesome Zombie sprite, but if you'll draw a tumbling rotting corpse, it's not exactly original, but can still be very technically good. In the other hand, someone can create an awesome new concept for a Zombie, but he lacks skill to make it look "pro", to put it in these terms.
All in all: The idea is that NO CONTESTANT is able to pull of perfect 10s on all three, because the work must be a balance of all parameters. If you're going to be incredibly original, you can't really adhere to the theme perfectly, and vice versa. However Skill is something else altogether, taking only the technicalities in consideration. This will allow everyone to get good rates in everything, but only a TRULY good mofo to win the contest, by being very balanced while being very skilled.
Or you know... The mods/judges can call this all bollocks :p
Sorry for the wall of text response.