Steps involved in entering a watermark like picture in our Excel worksheet:

• Click on the insert tab
• Select shapes
• Select rectangle
• We draw a rectangle around our data
• Right click on the rectangle
• Select the arrow next to the fill color bucket
• Select Picture…
• Select a picture of your choice from the My Pictures folder
• The picture is inserted
• Right click on the picture
• Select Format picture..
• In the Format Picture window we select fill and then arrange the transparency value to around 81%
• Next we click on the Line Color on the left-hand side and select ‘No line’
• Now we close the Format Window
• The picture covering the data looks like a neat water-mark

Further reading:
Insert a Watermark in Excel