This feature was removed in https://github.com/Guake/guake/pull/1028, with the suggestion to reimplement it after the new version hit a stable state.
Now that the new version is released, it would be nice if you could try to find a workaround for re-enabling it.
This is currrently the only thing holding me back from using Guake 3.
Thumbs up on this. -Ability for background image in guake terminal
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