If you have the original document, it's best to add the image there and then create a new PDF.
If not, do you want to add the image as a separate page by itself, or add it onto an existing page?
Adding the image as a new separate page is easier. Just drag your image onto Acrobat, not the PDF, but to an empty Acrobat application. Acrobat will automatically convert the image into a PDF. Then save the image pdf as a temporary file.
Open your document PDF, and just insert the image PDF where you want it in your original document PDF, and save.
This works best if your image is large so that it creates a near full page size of your document PDF.
If you want to add the image onto a page in the PDF, you can't just paste it onto the page. Content on a PDF are either text or objects. So to add an image, you have to create an object container and insert the image into the object container.
Open your PDF, and select the Touchup Object Tool:
Tools menu > Advanced Editing > Touchup Object Tool.
Or if you have the Advanced Editing toolbar open, it's the icon with the yellow pencil on a white square with text.
Go to the page where you want the image, and move your Touchup Object Tool cursor to the spot on the page you want the image.
Then right-click there, and on the context menu that appears, select "Place image". A window appears for you to select your image file and add it to the page.
With the Touchup Object Tool, you can also select the object container with your image and move it around the page to place it exactly where you want it.
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