I wanted to do the same - work out all the images people have referenced in their HTML file and upload them too: but you'll hit these problems:
You'll be able to use the IMG tag to find images in normal HTML, no problem, BUT what about Word docs people have converted to HTML? You'll have problems getting those image references out so easily.
Also, even if you do find out what all the images are, you can't grab them yourself - you'll have to ask the person doing the uploading to upload them all for you... and people will soon get fed up of doing that.
I ended up providing a text field for people to enter a URL to point to their webpages instead - if they've made a webpage, I'll let them reference it, else they can upload a normal Word or text doc... I basically don't let people upload html, php, asp, htm, - anything that could do damage or cause problems.
Seems restrictive but it makes sense.