I'm not sure how to select a specific page from a pdf, but this is what I use to convert single-page pdfs to jpgs:

convert -colorspace rgb input.pdf -antialias -quality 60 output.jpg
[download]

You really MUST use -antialias, and if you're converting CYMK pdfs to jpgs for the web, you really should also use the -colorspace rgb option, since many browsers suck at CYMK jpgs.