this can only be a general answer without knowing your blog url.
if your first background picture is NOT in the body {...} style, then you have the possibility to make yourself an image with the color grading you want -in photoshop or so- ( just a narrow vertical strip image - say named 'bg_gradient.jpg' - higher than your existing background image, but only a few pixel wide) and include that as background for the body (in the style.css):
body
{
background: url(path_to/bg_gradient.jpg) top repeat-x
}
'path_to' is the file path to your images in the theme directory.
oh, and to stop your background image from tiling, the declaration in the style sheet should be:
background: url(path_to/image.jpg) top center no-repeat;'
for more help, please post the link to your blog. ;-)