Favourite RPG Illustration
Just like letting the writing speak for itself, I like the artwork to do the same. I'm glad this wasn't "favourite RPG artist", because there are a few artists who consistently produce evocative stuff, but like the "author" question, it often requires good context, editing, layout, and other factors before these illustrations really shine.
There's another factor at work here, and that's the question of what an "illustration" is in regard to an RPG. Is it a diagrammatic representation of an RPG? (In which case I can think of very few pieces that accurately fit the description.) Is it a depiction of something found in game? (In which case there are far more examples, but are they better as stand-alone artworks, or as rpg imagery?) Is it something to inspire a character, or even a campaign around? (In which case I've had a few of those over the years.)
That's not even getting into a discussion of technical prowess of illustration skill versus raw emotional transmission.
One of the nicest artists I ever had the pleasure of meeting was Keith Parkinson, back at a comic convention in Sydney in the mid 1990s. One of his images that served as an inspiration to several of my characters, and a pair of campaigns was his image of "Shandara", which appeared in one of the colour plate sections in the Rifts core rules.
There was just something cool and bad-ass about this shamanic woman who was far more appropriately clothed than most women I'd seen in fantasy illustrations.
I still think it's a great image, but whether it's a good "RPG illustration", that's not so certain.
If we're working off the notion that an "illustration" is something that conveys specific information in a visual medium, then one of my favourites would be a more vague and nebulous entity known as "the height chart". Examples of two I like come from D&D 3/3.5.
You can describe heights of races in text, but it really doesn't mean much until you see a figurative representation of the different character types literally standing together. It conveys that data quickly and succintly, often with a bit of character and atmosphere thrown in.