Header Pixelation Issue!

Hi, I’ve been having issues when attempting to create my own header for my wordpress blog.
It seems to pixelate, even though i’ve created it in illustrator (which should be vector) and saved it for web as a high quality jpeg.
I used the correct image size for the theme i’m using which was 1280 x 300
however i’m having no luck..

Can anyone help me get to the bottom of this?

thanks

ste