+1  A: 

Depends where the css file is in relation to the image. Is the .css file in a folder off the root? If so try "../images/background.png"

redsquare
Please see my edit in the question. :D
Sergio Tapia
+1  A: 

Depending on your web-server, pathnames can be case sensitive, so it would be:

Images/background.png

Edit: If your Images folder is in the document root of the web-server, you can always use:

/Images/background.png
jeroen
+3  A: 

The background images you define inside CSS is always relative to the path of the CSS (if you do not explicitly say not to).

So, if you want to use:

background: url(images/background.png) repeat;

...the CSS-file must be located in the "parent"/root of the images-folder, so I suggest you move the Images folder into App_Themes/Default.

Mickel
Note the `url(...)`, it looks like you've forgotten that important piece.
jgeewax
Oh yeah, sorry. I actually think he should move the Image-folder into the App_Themes/Default so I'll leave it as it is.
Mickel
Thanks Mickel and JJG, your help worked. :)
Sergio Tapia
A: 

Try '../../images/background.png' if the folder structure remains the same as in your Solution Explorer.

treznik
Please see my edit, it's not working and I don't know why :(
Sergio Tapia