Here's how to remove the picture background or watermark from your slides.
On the View menu, select Master > Slide Master.
In the thumbnail pane on the left, click any slide layouts that have the picture watermark you want to remove.
On the Slide Master tab, click Background Styles.
Click Format Background.
In the Format Background pane, under Fill, click Solid Fill.
Click the Color button and select white, or any other color you like.
If you want to remove the background from all slides, click Apply to All.
See Also
Add background graphics or transparent pictures (watermarks) to your slides in PowerPoint for Mac