Treasures of Nadia has a really good tile when it shows up in the "Latest Updates" tab (AKA where I spend a disturbing amount of my day). Mythic Manor is another good one, its bright, shows off the characters, and catches the eye.
Now in terms of larger format stuff, I've always been partial to the stuff Designers Republic did back in the late 90's early 2000's, particularly the art for the Wipeout games. Very clean iconography, consistent design, and great use of whitespace.
Something I'd recommend being careful on is placing character art too close to the top/bottom of the image, as those tend to get cut off some. While on that subject, if you are doing a Renpy/RPGM based title, try to take into account that the bottom 20% or so of the screen is going to be obscured by a text box, so unless you want me prematurely wearing out my H key, don't put the action down there in crowded territory.