On a channel’s profile page (e.g. twitch.tv/USERNAME/profile), it seems that BTTV causes the images in the Info tab of the right sidebar to get clipped a little bit. These are the images from the channel’s custom panels, and while the images are 320px wide, they are only given 300px of space to fit in.
Normally, Twitch just scales them down to fit the smaller space, but as seen in this screenshot, BetterTTV uses a !important CSS rule to force them to be 320px, causing them to overflow their container and get cut off.
Here’s a comparison from my own profile of what the sidebar looks like with and without BTTV: http://imgur.com/a/B5dQj
My setup: BetterTTV v6.6, Chrome v40 (latest), Windows 7 64-bit.
