Not sure what your budget is but if you want something automated our cms (
www.elevatedx.com) has a transcoder that will encode videos and cut clips. It will also let you do what you described - take a video and automatically take a series of 300 or however many screencaps you want and make them into a gallery. Also watermarks pics ad videos, resizes, sharpens the pics, etc.
It works great and will do huge batch jobs but might be overkill for what you need to get done. If you want to check out the demo shoot me an email and I'll send you a login.
AJ