Most blogs load from top to bottom, left to right. A custom background code will often be the heaviest image on your blog. I don't know about you, but a background is the last thing I want my readers to see. I would rather my content load first, then my background.
Most sites instruct you to paste their background codes into an HTML gadget, but seldom do they encourage you to move the gadget once you've installed it. I suggest pasting the code into an HTML gadget, then moving the gadget to the very bottom of the rightmost sidebar. This will help your content load faster, which will give an impression of a faster loading blog. Click here for help on moving the gadgets in your sidebar. SUPER easy!
How To Install Your Own Background
Store Your Full Sized Background Images For Free!
Improve The Loading Time Of Your Blog
Awesome Links For Blog Help/Free Backgrounds
Rearrange Your Sidebar Gadgets