Dimensions for a website background image

I am confused in deciding the dimensions for a background image for my website. I am creating a collage like image in picasa and it defines the image dimensions using aspect ratios. If I want to create an image of 2048 x 1800, what aspect ratio should I use?

Wolfram Alpha gives the exact result 256/225, so an aspect ratio of 256:225 is correct. (Of course, a more inexact value would be 1.14:1, which would give you something like 2052x1800.)

As the aspect ratio is the ratio of the width of the image to its height, the aspect ratio of 2048x1800 image is 1.14:1 (2048/1800 = 1.14).