Back before the days of Google, I used to physically flip through magazines in search of pictures of girls (models in ads or pictorials, not celebrities) to use as inspiration for the looks of my female characters. Nowadays, I still try to use unknown models or lesser-known celebrities in banners so that people don't look at them and automatically think, "That's Paris Hilton." (I know no BSB fan would actually use Paris Hilton in a banner, so that seemed like a safe choice so as not to offend anyone LOL.) I want the opposite to happen. I want them to later see said lesser-known celeb in a movie or something and go, "Hey, that's Claire!" instead of "Hey, that's Ron Howard's daughter!" LOL.
I don't always use pictures of girls in banners, but if the story is predominantly about the relationship between a boy and a girl, it just makes sense to include both of them in the banner. It's not about attaching an image vs. letting readers imagine what she looks like; it's just about creating a banner that represents the story and looks pretty. I think the tendency for fanfic writers to use photos in their banners at all stems from the fact that we ARE writing about celebrities, so it makes sense to use real pictures of the Boys.
I will say, though, the trend with both banners and book covers seems to be moving towards simpler banners with images that are more symbolic than literal, and I like that. I hate to use it as an example, but the Twilight saga is a good one, as is the Fifty Shades trilogy. Scrolling through the Most Recent page just now, the banner for this story caught my eye:
http://absolutechaos.net/viewstory.php?sid=11103 It's simple, but effective: It's visually striking, eye-catching, and thought-provoking. Who does that hand belong to? If I wanted to read about the Boys' fictional kids being involved in a school shooting, I would click to find out.