Me, I usually end up with the mental image first, but that almost always leads to frustration as I can never find anything that comes close to matching it. So I tend to try to find inspiration first and then build around that. In the former case I almost always give up and go strictly with a written description, but having a portrait really, really helps me a lot when generating a character.
Here's a few of the ones I've used in past games. My group tends to favorite Rifts, but this really is open to any sort of game. Unfortunately, I didn't actually see a general forum that encompassed them all that wasn't specialized for something else, so I opted for the Rifts one. Apologies if there was somewhere more appropriate that I overlooked.
- Resarion, a wolfen cyber-knight. I was a bit disappointed at the lack of more modern trappings in the artwork, but the overall look was just too cool to my eyes.
- Dario, a really fun techno-wizard that lasted a lot longer than his unintentionally suicidal tendencies should have allowed. "Curiousity killed the techno-wizard" and all that.
- Kincade, a two-fisting, gunslinging-wannabe juicer. ChuckWalton does some amazing work with Rifts concepts and deserves all the praise. He was made for a game in the Vampire Kingdoms, and I just couldn't pass it up once I saw the picture. It was my first time trying out a Juicer, and they've gone on to become one of my most favorite concepts in the game.
- Arthur Hess, a combat cyborg and former sheriff from the New West who went with a Mark I CyberSlinger chassis. I thought he was going to be more fun than he was, but he ended up being completely outmatched by a mundane Gunslinger villain and perished in his first duel. Let's just say that I didn't mind rolling up a replacement character, hahaha.
- Cogsley, another techno-wizard for a more off-the-rails one-shot adventure, this one of an evil alignment. This one was a gnome, but we didn't bother trying to create a race for him and just used the typical human/normal d-bee stats. He wasn't as fun as Dario was, but I got to play around with a way more crazy TW items with him. So that was enjoyable.
So how about you? Where do you find your inspiration, and what kind of art do you tend to favor for your characters?