ZonePDF is an online service that does lots of handy conversions. For example, give it a big PDF file and it'll convert it into a zip file of images, with one image for each page of the original PDF file. Choose from 3 different image qualities. In my tests, a 150-page PDF file resulted in anything between 100MB and 700MB of zipped image files, depending on the quality I required.You can also convert Word and PowerPoint to PDF, and loads more options too.
The service appears to be free, but you'll need to sign up for an account. There appears to be an implication that you only need to sign up if you want to do more than 2 conversions, but that wasn't the case when I tried it. So if you can't get the conversions to work, sign up. That should fix it.