Using relative paths or file: paths to refer to images does not work with UIWebView. Instead you have to load the HTML into the view with the correct baseURL:
NSString *path = [[NSBundle mainBundle] bundlePath];
NSURL *baseURL = [NSURL fileURLWithPath:path];
[webView loadHTMLString:htmlString baseURL:baseURL];
You can then refer to your images like this:
<img src="https://stackoverflow.com/questions/747407/myimage.png">
(from uiwebview revisited)