Here's the problem: you have a PDF file. The source document is not available, but there is a last minute edit needed. You need an image added to the PDF. If the original document were at hand, you could open it up with the file's native application and import a new graphic where needed. Just re-convert to PDF with the image in place and your work is done.
Without the original document, you'll need to edit the PDF file itself. How can this be done? There is an Export command under Acrobat's File menu, but no Import. Hmmm... InDesign calls the command required to import an image onto a page 'Place'. But digging through Acrobat's menus doesn't uncover a Place command either.
Acrobat 9 Professional does have a Place command, but you won't find it under any menu. Here's the trick. With your document open, and with the spot you wish to add the image in your view panel, switch over to Acrobat's Touchup Object Tool (it's the right-most tool on the Advanced Editing Toolbar).
Your cursor will become a black arrow with a small square off to its side. Right-click to bring up a shortcut menu. THERE you will find the Place Image command.
Select Place Image and navigate through your file system for the desired image file.
For this example, I used a .png file to take advantage of the file format's transparency capabilities. Once the image is placed, it remains selected, scalable and moveable with the Touchup Object Tool. Once satisfied, you can switch over to the Hand Tool and click on the page to deselect the image. You can tweak the image at any time by using the Touchup Object Tool.