Here is a background I made for the custom boxes here on deviantart. This feature is for premium members only though.
To use this copy the following code and then paste it into a custom box on your profile:
<div class="popup2-moremenu"><div class="floaty-boat">
<img src="http@@://i211.photobucket.com/albums/bb190/andreamoore1985/bluebowbackground.png"/><@/div><@/div><div class="gr-box gr-genericbox"><div align="center"><div align="center">
Your box content goes here
Make sure you take out all of the @ symbols from the code other wise the code will not work!
You do not have to ask for permission to use this as long as you only use it on deviantart, you do not sell it (it's for personal use only) and that you make sure to credit me some where on your profile as the maker of the background.
Comment/fave if you use/plan to use