Open your Google Slides presentation
Select the slide you want to edit
Click Slide
Click Change background
Click Choose image or select a color
Upload or select the background you want
Click Done
To remove an image background, replace it with a solid color or a blank background
To make the slide background transparent, use an image editor before uploading
To remove a background from an inserted image, select the image
Click Format options
Use the Adjustments or Transparency settings if available
If needed, remove the image and insert a new one with a transparent background