If you're looking for a great Twitter cover picture I recommended checking out our wallpaper section.
However our wallpaper section of course doesn't cater to all audiences because we mainly have minimalistic and plain wallpapers in there. If you're looking for high quality Twitter cover pictures I recommend heading over to TwitrCovers.
The site was made purely to showcase Twitter cover pictures. They've got an absolutely huge list of cover pictures and you can even hover over each picture to see what a profile would look like on top of the picture. There are cover pictures to suit absolutely everyone whatever your taste.