Retrace your steps, start by checking your .bash_history to see what you deleted and so on
why do ppl like to mess around with system files when they have absolutely no idea what they're doing?
Besides that, there is no way people can help you by reading "I messed up in terminal" ... what exactly did you do and what exactly are the consequences. Just for any future posts you might write.
Luckily for you I looked up the location of that default image:
/System/Library/CoreServices/DefaultDesktop.jpg
so you can just copy another image to that location to get rid of that dreadful blue
cp <desired_image_path> /System/Library/CoreServices/