It's fairly easy to add your own image to the Steam UI using the Metro Skin, here's how to do it:


1. Find a wallpaper or a texture that you like
2. Make sure it's sized at least 1920x1080
3. Right click and select Open With Paint
4. Use the crop tool to select a portion you want, approximately 1920x120 (or about 1/9th of the image vertically, don't worry if it's bigger)
5. Save

6. Go to http://image.online-convert.com/convert-to-tga
7. Click "Convert"
8. Download the image
9. Rename it to clienttexture4.tga
10. Copy and replace to 

C:\Program Files\Steam\skins\WP7\Graphics
(or C:\Program Files (x86)\Steam\skins\WP7\Graphics if you are using 64bit)

11. Restart Steam