If you want to post a portion of a PDF file, you can use the Snapshot tool (Tools -> Selection Tools -> Snapshot Tool) and select and area.
Then, open Paint and go to Edit -> Paste
Then save the image as a .jpg file, which you can then upload
If you have Vista, you also have a snipping tool (Programs -> Accessories -> Snipping Tool). This allows you to select the area from any part of your screen and save it as an image in a single step.
