there is background picture that came out of blue and it does not go away?!

I have made a presentation slide in an A4 format and saved it as .pptx. The document looked nice. So I closed it and the next day when I opened it, there was a picture stick to the document (like a really low quality background right in the middle of the page) which I did not add and no matter what I do, it just doesn’t go away. Unfortunately I cannot upload the picture here. But any help is appreciated!