A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Just applying a new design may not work if the original was not professionally created.
If the authorr just added an image to the master or layout it is marked by default as userDrawn (Thank you Echo for teaching me this!). In essence this means "They want it do-not remove"
Go to View > Slide Master and look at each master and layout and try to delete the logo. Then apply one of the Microsoft designs.