(Based on V2.0.0 firmware or higher).
First put some jpg pictures in a folder.
Be sure they are not to big (500kb max).
Now browse with Media Library to your source folder and go into it.
Put the higlight on the desired jpg background picture and press the menu button on the remote control.
Select Set to Background and you will see the background changing.
If you want to reset to the default background, use the Restore Background selection in the menu.Always put your Mede8er to standby and start up again after a background change to clean memory and have stable performance.
You can download some backdrop examples here:http://www.mede8er.org/artwork/temp/X2_Backdrop_Samples.rar
Or just google for nice jpg pictures and save it as a .jpg file and load it as background.
Some screenshots of a Mede8er with custom background installed: