I am on a Mac and in Safari, all I see is black. It shows fine in Firefox and Opera.
I would question one of several things.
What is the format of the image in the PDF and is it a "good" image? Can you extract that image from your PDF and display it in any/every browser?
Is the image in your PDF in RGB or CMYK format?
Try doing the convert on a single image with a simple command line? Perhaps add -colorspace RGB.
I am not too sure of this, but try
convert -colorspace RGB image.pdf -resize 800x800 output.jpg
Try also some other format like PNG or GIF and see if that displays in any/every browser.
Someone like Anthony, may be able to give you more information about properly converting from PDF to JPG and any issues there. But see
http://www.imagemagick.org/Usage/formats/
(particularly about JPG and potentially about changing colorspaces and profiles, if that is relevant, and also other formats section regarding PDF). Note I believe IM uses Ghostscript to convert from PDF, so your Ghostscript may be an/the issue.
Hopefully, someone more knowledgeable, such as Anthony or Magick, can help you more directly.