What device are you using, mobile or webpage?

deendans: I use the webpage

you can try clicking your profile name on the top right hand corner
> Select Customize layouts
> Upload your own background image

If you wish to use css, you may try this code:
1. copy this code:
._lch_ {
background: url("<link of your image>");
}

2. change <link of your image> to your desired image (it can be an image uploaded to your own plurk post)
3. Paste the code above to the text box for css

deendans: Oh, thank you very much!!

hope it works!