yeah... that's the problem.
PDF files are not considered images (gif, png, jpeg, etc.)
Instead PDF files are documents.
If you want to display a picture instead, or alongside the PDF, then you'll need to create an image.
The easiest way is to show the image on your display, and capture it into your clipboard with Alt-PrintScreen... then open an editor like MSPAINT and paste it in from the clipboard. You can edit the image (crop, add arrows, etc.) then save to a file. PNG, GIF and JPEG files are good choices. Once you have a file, then upload it into the forum as described above.