You can define a background colour or use a graphic as a background for cell ranges in LibreOffice Calc.
Applying a Background Colour to a LibreOffice Calc Spreadsheet
Select the cells.
Choose Format - Cells (or Format Cells from the context menu).
On the Background tab page, select the background colour.
Graphics in the Background of Cells
Choose Insert - Image - From File.
Select the graphic and click Open.
The graphic is inserted anchored to the current cell. You can move and scale the graphic as you want. In your context menu you can use the Arrange - To Background command to place this in the background. To select a graphic that has been placed in the background, use the NavigatorNavigator.