(Nice pictures and all that).
Psychobabblelitphilosophically speaking, I don't really agree (with Oor and Tamar). Most people in SL obviously display some idealized self-image as their avatar (and 'person') in SL, if they have the energy and ability to get around to any idealizing. And even among the RP-ing crowd, most tend to play one character (or alts that might as well be one character) that is such a somewhat idealized self-image of them, or part of them. Most seem to go for an avatar that has at least SOME link to how they perceive themselves to be in RL. Older male players often go for an avi with gray hair, for example, because they want to appear somewhat like how they are in RL. Not all do, obviously. The short fat guy might play the muscular tall elf, and look totally different, but that doesn't mean that that avi isn't an expression of the typist, even if it is just an expression of what they perceive to be handsome, beautiful, interesting, compelling, etc. and most will still tend to have their avatar (and the character) have some connection with themselves. Even in serious RP sims, most players tend to play characters that somehow are really 'them. Maybe that character is a wayward Prince that has killed his brother, and not the loving father and husband and social worker that the character is in RL, most likely, in his RP, that player will at least partially play his character as himself, gravitate towards making the choices he himself might have made in situation X, and so forth, most likely refer to his character as 'me' in most OOC conversations, just as most people playing a video game will likely think of their controlled character as 'me', even if that character is a fantasy Witcher or a criminal in GTA. People identify with protagonists in fiction, and they identify even more with characters they control in roleplaying.
And even if you are talking about that minority of 'serious' RP-ers that really make an effort at playing characters that are distinct from themselves, are not in any way connected to them, they still draw on their own mind to supply those characters with the right make-up needed for them to to be anything but a flat cliché. It is the same in fiction. Often enough, you see the writer come up with character that they obviously have no real connection to. Like the evil-doer that is just paper-thin, without any depth, because the writer just could not or didn't bother to 'go there'. That is often what separates good writing from bad, good characters from bad ones. You can try to play a character so-and-so, but unless you can 'feel them', get in their head, occupy their head, temporarily, they will be off or just superficial. So, the smart writer tends typically tends to limit himself to more detailed descriptions and perspectives that he or she CAN get 'into'. It is probably a good thing that Stephen King typically doesn't employ female protagonists. And that Lena Dunham didn't make a TV series called 'Boys'.
And typically, in SL, among those RP-ers that really treat their characters as separate entities from themselves, this separation is often a lot less total than they imagine it to be. Sure, the shrewd Schendi slaver might be a very different character with a very different background from the proud Torvalslander they played before, but, during RP, an outside observer may notice that they act and talk rather similarly, do the same sort of thing, etc. Which only makes sense, because they are powered by the same set of brains with the same wiring. People tend to VASTLY over-estimate the apparent differences between their avatars and characters, just as people with alts often tend to think that they are very different (and will often deny being the alt of the other avi), while it is blatantly clear to the casual observer it is the same person.
And hey, sure... there might be a VERY SMALL group of RP-ers that really DO manage to maintain some iron wall of IC/OOC character separation, that manage to play different characters that make sense, don't have much in common, without having to 'crawl into their skin' themselves. Not every actor is a method actor either, after all. But even in those cases, that character will STILL be an artistic expression of them, and particular to them.
Take those (nice) pictures of your (Oor's) avatars. They certainly don't look the same. But... is there a theme? A similarity in style? Of course. Some between the avatars, and some because the person that took the pictures was you (if I go by the descriptions). And nothing wrong with that, either.
Anyway, I am sure that Darby meant something like 'express your personality or your vision of the character you want to play'. Doesn't really matter. Whether making an avi that is supposed to represent you, or making one that represents a fictional character that hasn't the slightest thing in common with you, I'd say that you want that avatar to be unique and 'your own', in as far as possible.