Sounds like the "background" is an image that has been inserted and sent to the back on all slides. This is NOT the same thing as a correctly inserted background and difficult to get out of.
If you have the image as a file:
Right click a blank are choose Format background and Picture/Texture and then FILE from the choices choices. Navigate to the image file and apply to all.
BUT if the slides already have a "sent to back" image you will have to delete them all!
This macro might do that for you (depending on what has been done)
Make sure you work on a copy
Sub fixFakeback()
Dim osld As Slide
For Each osld In ActivePresentation.Slides
If osld.Shapes(1).Type = msoPicture Then osld.Shapes(1).Delete
If osld.Shapes(1).Type = msoPlaceholder Then
If osld.Shapes(1).PlaceholderFormat.ContainedType = msoPicture Then osld.Shapes(1).Delete
End If
Next
End Sub
How to use a macro