The whole dashboard’s background? Not at the moment, but you might be interested in casting a vote for:
I suspect that if we put in the work to support variables in Media Tiles, the same concepts would transfer over to dashboard backgrounds.
You might also be interested in the Weather Widgets: Custom Tile collection by @Michael – there’s some neat options in there that can display different background images (in a weather tile) based on the weather and a new weather clock that displays conditions.