How To Add A Background Image In Gmail ?

You mean to add image in your email.if yes then you need to
1)Go to settings
2)Select Labs
3)Enable “Inserting images”
4)Save changes.

Now if you wish to add image in signature.follow these steps
1)Go to settings
2)under the general tab find Signature
3)For inserting an Image in the signature.first you have to upload the image in free image hosting sites.

for eg:go to imageshack.us. browse for the image you want to upload.and click upload.after its uploaded,copy the direct link

4)There is an image icon in the signature box.click on that and paste the direct link you copied from imageshack.us

That it. have a nice day