Answer 1

background image tag?
no but there is background-image property.

set background image as follows.
body
{
background-image:url("here_goes_url_of_image");
}

for more info refer this :
https://www.sololearn.com/learn/CSS/2247/?ref=app
https://www.sololearn.com/learn/CSS/1105/?ref=app
https://www.sololearn.com/learn/CSS/1104/?ref=app
https://www.sololearn.com/learn/CSS/2244/?ref=app