<Note: thanks to Buffy Miller for giving me a much slicker solution here>
Often I want to post a PDF on a web page, but would like the cover of the PDF to be the clickable element. This just makes things look slicker. This can easily be done with the imagemagick set of commands. Using imagemagick, you can select the page you want to convert out of the pdf (using the  annotation), get the right size (using -resize) and format (based on file extension). The following command does the conversion in a single go:
convert -resize 150 inputfile.pdf outputfile.png
One of these days I’ll get around to making a mediawiki extension that does that by default for pdf media attachments.