you should link the URL to your template directory
create a "images" folder in your template directory (or if there is a images folder use this one) upload your logo there
use src="images/header.jpg"
other thing you could try is uploading your header image in your media folder
in your index.php you now use:
<img src="../../media/header.jpg">
in CSS:
the color attribute is for the TextColor
so if you add the line:
color: #000;
the color of the text will be black
background is used for backgrounds (no s***....)
you can add all sorts of info in the "background"
you can also use "background-color: #000;" witch only specifies the color

**just typed everything without checking anything.. so if it isn't working properly, srry =D**