If your not using a custom css, here's a way:
1) Take a picture of the character in Brawl (the whole body not just the head)
2) Convert the picture into a JPEG file (unless you decided to take it in BrawlVault)
3) Use Gimp or Photoshop and add transparency around the character. Then save as a PNG file.
4) Search up "Brawl Background" on Google. Find the one that has the background without a character but has a shadow. Copy.
5) Paste it in a new document of Gimp/Photoshop. Save if you want.
6) Copy the character and put him on top of the background. Try to position him/her so that his feet touch the base of the shadow. Again, save if you want.
7) Export any one of the CSS icons from BrawlBox to your desktop.
8.) Take the head area of your character in the background and copy it. Paste it over the CSS icon.
9) In BrawlBox replace the character who you're replacing's CSS icon with the new one.
10) Save the new CSS to your SD card.
11) You now have an updated CSS!