There is no easy way. Regex the HTML using RegexKitLite (http://regexkit.sourceforge.net/RegexKitLite/index.html) and snag all the urls to .jpg,.gif,.png, and .css and .js and whatever all else you need.
alternately, call:
NSString* imgUrls=[webView stringByEvaluatingJavaScriptFromString:@"document.getElementsByTagName('img')"]
or something like that, I'm no javascript whizz... and then deal with whatever all that returns ;)
Sorry. It's a pain in the rearheinie.
edit:
Save all the img's on the iphone, also save the html file. When you want to reload the page, load the html from a file into a string, and then use
- (void)loadHTMLString:(NSString *)string baseURL:(NSURL *)baseURL
to load the HTML string. baseURL is used to specify the directory or site the webview will imagine the html string you hand it is located. All URLS will be relative to that.
Note, of course that this will not work very well for absolute URLs, only for relative ones. So this, in your html file, will monkey things up:
<img src="http://google.com/f/r/i/g/img.gif">
while this would be ok:
<img src="f/r/i/g/img.gif">
Again, this whole solution is mucky.
You might look into a pre-existing open source recursive html spider. I think wget does what you want, but I doubt it can be compiled for iPhone without a -lot- of hassle.