How to change terminal background picture on Ubuntu 18.04?

Sadly, this option no longer exists. See Bug 698544:

• Comment 1 (2013-04-22) is Christian Persch's (rather incomplete) answer to the bug report.

  No.

• Comment 45 (2013-05-13) by Allan Day provides further details.

  I've spoken to Christian and he has filled me in on the background to this issue.

  The ability to set background transparency was removed as a part of a much larger clean up of the gnome-terminal code base. This modernisation effort is much needed and it should be remembered that the terminal has an old and complicated code base. Background transparency was also known to cause a range of performance issues, including slowness and memory leaks.

  Given the number of active terminal developers, the need to clean up the code base and known bugs with background transparency, it was decided that the resources were not available to maintain background transparency and keep the quality to the required level.

  We understand that some users are disappointed by the disappearance of this feature, and we hope that they understand that this decision was made to enable other improvements to the terminal code, and that it will lead to other improvements in the long run.

  While Christian’s initial response could have been better, he has responded in detail in private mail and I suspect that he was simply tired of the issue.

_{* Bold is mine.}

_{** This answer makes Persch sound like an arsehole, but that is not my intention. Read Day's last sentence for context.}

you can change it as follow

open a terminal --> Edit --> preferences

Then in Color tab, you have to uncheck: use color from system theme

in this way, you will be able to change terminal background