An update is coming that can help with this. There are two factors happening here though...
1 - the standard GD library used on the server doesn't do a great job resizing images and reproducing color. It's a problem that we can not control. Which is why we will be releasing an update that uses a different image handling library.
2 - in some cases people can improve image quality by changing how they output them from their app. This is a bit complicated and I've never understood what people were really doing.
Your background image is not resized which is why it looks identical to the original.