A background image in PowerPoint can set the theme of a presentation. You can add a background image for your entire presentation or a single slide. Follow these steps:
- Go to the Design tab and click Format Background.
Click Format Background. - A “Format Background” panel will appear on the right. Click Picture or texture fill, then click Insert…
Picture or texture fill > Insert… - Find your image from your computer’s files, then click Insert.
Find your image, then click Insert. - Optionally, drag the Transparency slider to the right to change the image’s opacity.
Drag the Transparency slider to the right. - The background image will now be added to your selected slide. You can also click Apply to All to apply the background image on every slide, then click the Close icon.
Click Apply to All. 
Check your slides to preview the background image.